The effects of coverage by an underpotentially deposited copper submonolayer on hydrogen adsorption were examined for three surfaces ((955), (322) and (544)) of platinum single crystals. A very small amount of copper preferentially blocks the hydrogen adsorption to the (100) step sites. This result supports our previous conclusion (J. Electroanal. Chem., 386 (1995) 75) that the most positive peak of copper underpotential deposition relates to the deposition of copper on the (100) step sites of these three surfaces.