Details of Nafion/polypyrrole composite formation have been obtained using electronic absorption spectroscopy. Three distinct process-monomer loading, polymerization, and removal of unreacted monomer-have been studied. Pyrrole loading displays a square root dependence on time, indicative of a diffusion controlled partitioning process. Partitioning of pyrrole into Nafion, however, is complicated by protonation of pyrrole and acid-catalyzed oxidation to give oligomeric and polymeric species. These processes are affected by the presence of oxygen and are photosensitive. A variety of oxidizing agents have been used to effect polymerization including Fe3+, H2O2, ammonium persulfate, and UV irradiation.