diff --git a/doc/sphinx/cython/importing.rst b/doc/sphinx/cython/importing.rst
new file mode 100644
index 000000000..fcb7763d4
--- /dev/null
+++ b/doc/sphinx/cython/importing.rst
@@ -0,0 +1,18 @@
+.. py:currentmodule:: cantera
+
+Importing Phase Objects
+=======================
+
+These classes are composite representations of a substance which has
+thermodynamic, chemical kinetic, and (optionally) transport properties.
+
+.. autoclass:: Solution
+
+.. autoclass:: Interface
+
+.. autoclass:: DustyGas
+
+Utility Functions
+-----------------
+
+.. autofunction:: addDirectory
diff --git a/doc/sphinx/cython/index.rst b/doc/sphinx/cython/index.rst
index 6472041fe..b9589104a 100644
--- a/doc/sphinx/cython/index.rst
+++ b/doc/sphinx/cython/index.rst
@@ -6,6 +6,7 @@ Contents:
 .. toctree::
    :maxdepth: 2
 
+   importing
    thermo
    kinetics
    transport
diff --git a/interfaces/cython/cantera/composite.pyx b/interfaces/cython/cantera/composite.pyx
index 0ce7b7d93..c111726e5 100644
--- a/interfaces/cython/cantera/composite.pyx
+++ b/interfaces/cython/cantera/composite.pyx
@@ -1,8 +1,32 @@
 class Solution(ThermoPhase, Kinetics, Transport):
-    pass
+    """
+    A class for chemically-reacting solutions.
+
+    Instances can be created to represent any type of solution -- a
+    mixture of gases, a liquid solution, or a solid solution, for
+    example.
+
+    Class Solution derives from classes `ThermoPhase`, `Kinetics`, and
+    `Transport`.  It defines very few methods of its own, and is provided so
+    that a single object can be used to compute thermodynamic, kinetic, and
+    transport properties of a solution.
+    """
 
 class Interface(InterfacePhase, InterfaceKinetics):
-    pass
+    """
+    Two-dimensional interfaces.
+
+    Instances of class Interface represent reacting 2D interfaces
+    between bulk 3D phases. Class Interface defines no methods of its
+    own. All of its methods derive from either `InterfacePhase` or
+    `InterfaceKinetics`.
+    """
 
 class DustyGas(ThermoPhase, Kinetics, DustyGasTransport):
-    pass
+    """
+    A composite class which models a gas in a stationary, solid, porous medium.
+
+    The only transport properties computed are the multicomponent diffusion
+    coefficients. The model does not compute viscosity or thermal conductivity.
+
+    """
diff --git a/interfaces/cython/cantera/utils.pyx b/interfaces/cython/cantera/utils.pyx
index 634181c2e..77f2ba162 100644
--- a/interfaces/cython/cantera/utils.pyx
+++ b/interfaces/cython/cantera/utils.pyx
@@ -17,6 +17,7 @@ cdef pystr(string x):
         return s.decode()
 
 def addDirectory(directory):
+    """ Add a directory to search for Cantera data files. """
     CxxAddDirectory(stringify(directory))
 
 def _have_sundials():