The catalytic activity for CO2 reduction was studied on Pt(S)-[n(111)x(111)] single crystal electrodes in H2SO4 solutions by voltammetry. Reduction rates on Pt(110), Pt(331) and Pt(221) were lowered remarkably above 0.15 V vs. RHE in 0.5 M H2SO4 compared with those in 0.1M HClO4. Activities on Pt(332), Pt(997) and Pt(111) in H2SO4 were, however, comparable with those in 0.1 M HClO4. Voltammograms on Pt(110), Pt(331) and Pt(221) in 0.1M HClO4+5x10(-4) M H2SO4 gave sharp redox peaks at 0.175V vs. RHE, corresponding to the adsorption and desorption of sulfuric acid anion. The potentials at which reaction was prevented were approximately equal to the beginning of the adsorption of sulfuric acid anion. The charge of the redox peaks at 0.175 V vs. RHE was correlated with the step density, Hindrance of CO2 reduction is discussed on the basis of the possible adsorption site of sulfuric acid anion.