diff --git a/exptool/io/psp_io.py b/exptool/io/psp_io.py
index f18535e..430dbec 100644
--- a/exptool/io/psp_io.py
+++ b/exptool/io/psp_io.py
@@ -30,6 +30,33 @@
 except ImportError:
     raise ImportError("You will need to 'pip install pyyaml' to use this reader.")
 
+def _to_python(obj):
+    """
+    Recursively convert numpy types to native Python types.
+
+    This helper function ensures that objects containing numpy types (such as numpy scalars or arrays)
+    are converted to their native Python equivalents. This is particularly important for YAML serialization,
+    which may not handle numpy types correctly.
+
+    Parameters
+    ----------
+    obj : any
+        Any Python object, potentially containing numpy types (e.g., numpy scalars, arrays, or nested structures).
+
+    Returns
+    -------
+    out : any
+        The input object with all numpy types converted to native Python types.
+    """
+    if isinstance(obj, dict):
+        return {k: _to_python(v) for k, v in obj.items()}
+    elif isinstance(obj, list):
+        return [_to_python(v) for v in obj]
+    elif hasattr(obj, 'item'):   # catches numpy scalars
+        return obj.item()
+    else:
+        return obj
+
 
 class Input:
     """Input class to adaptively handle OUT. format specifically
@@ -79,8 +106,7 @@ def __init__(self, filename,comp=None,verbose=0):
         # do an initial read of the header
         self.primary_header = dict()
 
-        # initialise dictionaries
-        self.comp_map       = dict()
+        # initialise dictionary for headers
         self.header      = dict()
 
         self._read_primary_header()
@@ -88,18 +114,68 @@ def __init__(self, filename,comp=None,verbose=0):
         self.comp = comp
         _comps = list(self.header.keys())
 
+
         # if a component is defined, retrieve data
         if comp != None:
-            if comp not in _comps:
-                raise IOError('The specified component does not exist.')
+
+            # or check if we are reading all components
+            if comp == 'all':
+                self.data = dict()
+                for c in _comps:
+                    self.data[c] = self._read_component_data(self.filename,
+                                                             c,
+                                                             self.header[c]['nbodies'],
+                                                             int(self.header[c]['data_start']))
 
             else:
-                self.data = self._read_component_data(self.filename,
-                                                      self.header[self.comp]['nbodies'],
-                                                      int(self.header[self.comp]['data_start']))
+                if comp not in _comps:
+                    raise IOError(f'The specified component, {comp}, does not exist.')
+                
+                else:
+                    self.data = self._read_component_data(self.filename,
+                                                        self.comp,
+                                                        self.header[self.comp]['nbodies'],
+                                                        int(self.header[self.comp]['data_start']))
+                    
         # wrapup
         self.f.close()
 
+    def write(self, filename):
+        """
+        Write the current data to a PSP/OUT. file.
+
+        Parameters
+        ----------
+        filename : str
+            The output filename to which the data will be written.
+
+        Behavior
+        --------
+        Writes all components to the specified file if `comp='all'` was used when reading.
+        Writing of single components is not implemented and will raise an exception.
+
+        Exceptions
+        ----------
+        NotImplementedError
+            Raised if attempting to write when `comp` is not 'all'.
+
+        Example
+        -------
+        >>> inp = Input("input.OUT", comp="all")
+        >>> inp.write("output.OUT")
+        """
+        if self.comp != 'all':
+            raise NotImplementedError("Writing single components is not implemented yet. Use comp='all'.")
+        
+        with open(filename, 'wb') as f:
+            self._write_primary_header(f)
+
+            # Now write all component headers and data sequentially.
+            for comp in self.header:
+                self._write_component_header(f, self.header[comp])
+                self._write_component_data(f, comp, self.data[comp])
+
+
     def _read_primary_header(self):
         """read the primary header from an OUT. file"""
 
@@ -117,6 +193,25 @@ def _read_primary_header(self):
             next_comp = self._read_out_component_header()
             data_start = next_comp
 
+    def _write_primary_header(self, f):
+        # time is always <f8
+        np.array([self.time], dtype='<f8').tofile(f)
+
+        # total nbodies and number of components
+        total = sum(self.header[c]['nbodies'] for c in self.header)
+        ncomp = len(self.header)
+        np.array([total, ncomp], dtype=np.uint32).tofile(f)
+
+        # next the component headers must follow immediately
+        # magic number goes at byte 16
+        # TODO: Writing the magic number is required for full PSP file compatibility,
+        # but this is not yet implemented. Uncomment and verify the following lines
+        # when ready to support the magic number in the output file format.
+        #f.seek(16)
+        #magic = 2915019716 if self._float_len == 4 else 0
+        #np.array([magic], dtype=np.uint32).tofile(f)
+        # double up the magic number for consistency
+        #np.array([magic], dtype=np.uint32).tofile(f)
     def _summarise_primary_header(self):
         """a short summary of what is in the file"""
 
@@ -133,6 +228,7 @@ def _read_out_component_header(self):
 
         #_ = f.tell()  # byte position of this component
 
+
         if self._float_len == 4:
             _1,_2, nbodies, nint_attr, nfloat_attr, infostringlen = np.fromfile(self.f, dtype=np.uint32, count=6)
         else:
@@ -161,6 +257,8 @@ def _read_out_component_header(self):
         head_dict['nbodies']     = nbodies
         head_dict['data_start']  = comp_data_pos
         head_dict['data_end']    = comp_data_end
+        head_dict['info_len']     = infostringlen
+        head_dict['info_str']     = head_normal
 
         self.header[head_dict['name']] = head_dict
 
@@ -169,9 +267,39 @@ def _read_out_component_header(self):
             self.indexing = head_dict['parameters']['indexing']
         except:
             self.indexing = head_dict['indexing']=='true'
+            head_dict['parameters']['indexing'] = self.indexing
 
         return comp_data_end
 
+    def _write_component_header(self, f, compdict):
+        """
+        compdict is one of self.header[name]
+        """
+
+        nbodies     = _to_python(compdict['nbodies'])
+        nint_attr   = _to_python(compdict['nint_attr'])
+        nfloat_attr = _to_python(compdict['nfloat_attr'])
+
+        compdict_clean = _to_python(compdict)
+
+        info        = yaml.safe_dump(compdict_clean)
+        info_bytes = info.encode()
+        info_len   = len(info_bytes)
+
+        if self._float_len == 4:
+            # the 6 numbers at the start: magic number + 5 (so only write 5)
+            magic = 2915019716
+            arr = np.array([magic, 0, nbodies,
+                            nint_attr, nfloat_attr, info_len],
+                            dtype=np.uint32)
+        else:
+            arr = np.array([nbodies, nint_attr,
+                            nfloat_attr, info_len],
+                            dtype=np.uint32)
+
+        arr.tofile(f)
+        f.write(info_bytes)
+
 
     def _check_magic_number(self):
         """check the magic number to see if a file is float or double"""
@@ -189,12 +317,12 @@ def _check_magic_number(self):
 
 
 
-    def _read_component_data(self,filename,nbodies,offset):
+    def _read_component_data(self,filename,comp,nbodies,offset):
         """read in all data for component"""
 
         dtype_str = []
         colnames = []
-        if self.header[self.comp]['parameters']['indexing']:
+        if self.header[comp]['parameters']['indexing']:
             # if indexing is on, the 0th column is Long
             dtype_str = dtype_str + ['l']
             colnames  = colnames + ['id']
@@ -202,13 +330,13 @@ def _read_component_data(self,filename,nbodies,offset):
         dtype_str = dtype_str + [self._float_str] * 8
         colnames = colnames + ['m', 'x', 'y', 'z', 'vx', 'vy', 'vz', 'potE']
 
-        dtype_str = dtype_str + ['i'] * self.header[self.comp]['nint_attr']
+        dtype_str = dtype_str + ['i'] * self.header[comp]['nint_attr']
         colnames = colnames + ['i_attr{}'.format(i)
-                               for i in range(self.header[self.comp]['nint_attr'])]
+                               for i in range(self.header[comp]['nint_attr'])]
 
-        dtype_str = dtype_str + [self._float_str] * self.header[self.comp]['nfloat_attr']
+        dtype_str = dtype_str + [self._float_str] * self.header[comp]['nfloat_attr']
         colnames = colnames + ['f_attr{}'.format(i)
-                               for i in range(self.header[self.comp]['nfloat_attr'])]
+                               for i in range(self.header[comp]['nfloat_attr'])]
 
         dtype = np.dtype(','.join(dtype_str))
 
@@ -228,3 +356,30 @@ def _read_component_data(self,filename,nbodies,offset):
         del out  # close the memmap instance
 
         return tbl
+
+    def _write_component_data(self, f, comp, data):
+        compdict = self.header[comp]
+
+        dtype_str = []
+        colnames  = []
+
+        if compdict['parameters']['indexing']:
+            dtype_str.append('l')
+            colnames.append('id')
+
+        dtype_str += [self._float_str] * 8
+        colnames  += ['m','x','y','z','vx','vy','vz','potE']
+
+        dtype_str += ['i'] * compdict['nint_attr']
+        colnames  += [f'i_attr{i}' for i in range(compdict['nint_attr'])]
+
+        dtype_str += [self._float_str] * compdict['nfloat_attr']
+        colnames  += [f'f_attr{i}' for i in range(compdict['nfloat_attr'])]
+
+        dtype = np.dtype(','.join(dtype_str))
+
+        outarr = np.zeros(compdict['nbodies'], dtype=dtype)
+        for i, key in enumerate(colnames):
+            outarr[f'f{i}'] = data[key]
+
+        outarr.tofile(f)