The kinetics of O-2 reduction on both oxidized and reduced Pt rotating disk surfaces covered with thin films (1 to 15 mu m) of recast polybenzimidazole (PBI) or Nafion have been examined in 0.1 M aqueous solutions of H3PO4, H2SO4, and HClO4 at 25.0 degrees C. Polymer films were deposited on a Pt disk electrode by a spin-coating technique. Experiments were performed formed at different rotation rates and potential scan rates. This investigation was undertaken to establish to what extent the solid polymer electrolyte influences the mechanism and kinetics of O-2 reduction. The number of electrons exchanged per O-2 molecule and kinetic parameters for O-2 reduction (reaction order with respect to O-2, Tafel slope/transfer coefficient, and exchange current density) are not changed in the presence of the polymer films, indicating that the reaction path and kinetics are not altered in the presence of recast PBI and Nafion films. The diffusion coefficient (D) and concentration (C) of O-2 in recast films were determined by means of linear sweep voltammetry measurements in quiescent solution. The D and C values in polymer films immersed in acidic solutions were found to have different values than those in the solutions. D(O-2) is about 2.5 times lower in the recast films than in the solutions, whereas C(O-2) is slightly higher in the film. The largest concentration increase, by a factor of two, was found in PBI film in 0.1 M HClO4. Cyclic voltammetric measurements under argon atmosphere have shown that PBI and Nafion recast films are electrochemically stable in the potential range between the onsets of H-2 and O-2 evolution. A decrease in the effective electrocatalyst surface area due to deposition of solid polymer films was 15 to 20%.