The reduction of H+ cations on the Pt electrode coated with poly(N-methylpyrrole) (PNMP) film was studied in an acidic aqueous solution. The reduction depends strongly on the counteranions originally doped in PNMP during polymerization. H+ cations can penetrate the neutral or overoxidized film from PNMP(TsO(-)), in which the H+ diffusion coefficient is estimated as 10(-5) cm(2) s(-1) by the potential step chronoamperometry method. But the neutral or overoxidized film from PNMP(NO3-) is closed for H- penetration.