Electrochemical oxidation/reduction reactions of ethanol in 1 M HClO4 were studied on porous Pt, Pt0.92Ru0.08, Pt0.85Ru0.15, and Ru under potentiodynamic conditions. The electrodes with defined bulk and surface compositions were made by electrodeposition on porous Au substrates. Cyclic voltammetry in combination with on-line mass spectrometry (DEMS) was employed to correlate faradaic currents with ion currents associated to reaction products. The formation of both CO2 and ethanal was unequivocally identified during ethanol oxidation on PtRu and Pt by using d3-ethanol. On pure Ru neither faradaic currents were observed nor oxidation/reduction products could be detected by mass spectrometry. The reduction of ethanol on Pt and PtRu to methane and ethane was found in the potential range of hydrogen adsorption. The yields of oxidation and reduction products and the corresponding onset potentials for their formation are greatly influenced by the Ru content. Alloys with high Ru content exhibit a lower reaction r...