Rotating disk voltammetry of ethanol at PtRu black and carbon supported Pt (Pt/C) catalyst layers has been investigated in 1 M sulfuric acid. When thick catalyst layers were used, increasing the rotation rate resulted in significant increases in current and produced linear Koutecky-Levich plots. However, a mass transport limited current was not reached due to inhibition of ethanol oxidation by oxide formation at high potentials. For PtRu black, the slopes of Koutecky-Levich plots gave an average number electrons transferred (n(av)) of 3.6 that was independent of potential and indicated that the predominant product was acetic acid. Similar n(av) values were obtained for Pt/C at high potentials, but decreased at potentials below 0.78 V vs. SHE, where the Koutecky-Levich plots began to show significant curvature. Non-linear Koutecky-Levich plots were observed at all potentials for thinner Pt/C layers, and accurate n(av) values could not be obtained. This appears to be due to an increase in the kinetic current with rotation rate. Since acetaldehyde is known to inhibit ethanol oxidation at Pt, its convective removal appears to be responsible for the increased kinetic current. (C) 2016 Elsevier Ltd. All rights reserved.