CO adsorbed on Pt(S)-[n(1 1 1) x (1 1 1)] electrodes was studied using infrared reflection absorption spectroscopy (IRAS) in 0.1 M HClO4 saturated with CO. The configuration of adsorbed CO depends on the terrace width remarkably: on-top and threefold CO are co-adsorbed on the surfaces with terrace atomic rows (n) more than nine, whereas only on-top CO appears on the surfaces with n less than eight, on which no multi-bonded CO was observed. Integrated intensity of the IRAS band and peak shift (dv/dE) do not depend on the crystal orientation.