Adsorption of sulfuric acid anion (HSO4- or SO42-) was studied using IRAS (infrared reflection absorption spectroscopy) on Pt(S)-[n(111) x (111)] and Pt(S)-[n(100) x (111)] electrodes in 0.05 M H2SO4. Pt(S)-[n(I 11) x (I 11)] electrodes give two IRAS bands. One is from a sulfuric acid anion adsorbed on the terrace with 3-fold symmetry (similar to1200 cm(-1)), and another is from that on the step with 2-fold symmetry (similar to1200 cm(-1) and similar to 1100 cm(-1)). Relative band intensity from the 2-fold sulfuric acid anion (similar to 1100 cm(-1)) gets higher with the increase of step atom density. Pt(S)-[n(100) x (111)] electrodes also show two IRAS bands around 1200 and 1100 cm(-1) which originate from the sulfuric acid anion adsorbed on the terrace and the step with 2-fold symmetry. The relative intensity of the lower frequency band increased with the increase of the step atom density.