Oxygen reduction on platinum single crystals has been studied in perchloric and sulfuric acid media using a hanging meniscus rotating disk electrode configuration. The surfaces studied belong to the [01 (1) over bar] zone and can be classified in two different series, i.e., surfaces with terraces with (111) symmetry and (100) steps and surfaces with (100) terraces and (111) steps. In sulfuric acid media, the highest catalytic activity is found for the Pt(211) electrode whereas the Pt(111) electrode exhibits an anomalously low j(0) value. Conversely, the extrapolated value for the catalytic activity of Pt(111) obtained from the series of stepped surfaces having (111) terraces is much higher than that obtained experimentally. The comparison with the results obtained in perchloric acid solutions indicates that the anomalously low catalytic activity of the Pt(111) electrode in sulfuric acid solutions is related to the formation of an ordered adlayer of specifically adsorbed (bi)sulfate anions. For practical purposes, the effect of the step density and the (bi)sulfate specific adsorption is small, except for the Pt(111) electrode. (C) 2003 Elsevier B.V. All rights reserved.