The binding positions (terraces, steps) and sites (on-top, bridge-bonded, threefold) of adsorbed CO have been studied on Pd(S)-[n(1 11) x (111)] (n = 2, 3, 5, 9, 20, infinity) and Pd(S)-[n(100) X (111)] (n = 2, 5, 9, infinity) electrodes in 0.1 M HClO4 saturated with CO using infrared reflection absorption spectroscopy (IRAS). The band of threefold CO is observed around 1950 cm(-1) on Pd(S)-[n(111) x (111)] electrodes with terrace atomic rows n >= 5. The band intensity is enhanced with the increase of the terrace width; threefold CO is adsorbed on the terrace of Pd(S)-[n(111) x (111)] series. Threefold CO is not found on Pd(S)-[n(100) x (111)] series. On-top CO is observed around 2070 cm(-1) on both Pd(S)-[n(111) x (111)] and Pd(S)-[n(100) x (111)] series. The band intensity is enlarged with the increase of the step atom density, supporting that on-top CO around 2070 cm(-1) is adsorbed at steps. A high frequency band of on-top CO adsorbed on the terrace is found around 2100 cm(-1) on Pd(S)-[n(111) x (111)] surfaces with n >= 20. Bridge-bonded CO gives a predominant IRAs band around 1980 cm(-1) on all the surfaces. The band intensity of bridge-bonded CO does not correlate with the step atom density; bridge-bonded CO is adsorbed on both terrace and step sites. The on-top CO at steps has higher Stark tuning rate (dv/dE) than the one at terraces on Pd(S)-[n(111) x (111)] electrodes. (c) 2005 Elsevier B.V. All rights reserved.