Measurement of the stoichiometry of ethanol oxidation is crucial in the development of direct ethanol fuel cell technology because it determines how efficiently the fuel is used. Here, it has been determined in fuel cell hardware at 80 degrees C for various anodes prepared with commercial Pt/C, PtRu/C and PtSn/C catalysts. Average numbers of electrons transferred per ethanol molecule (n(av)) obtained from the flow rate dependence of the current (electrochemical method) are critically compared with values obtained from the amount of ethanol consumed and yields of acetaldehyde, acetic acid, and carbon dioxide (chemical analysis). In general, the electrochemical method provides accurate values of n(av). Crossover of ethanol to the cathode causes a small (<10%) underestimation of n(av), but this is generally less than the uncertainly of the measurement. However, when yields of carbon dioxide are high (>50%), the dependence of n(av) on the ethanol concentration can lead to its overestimation, and to impossibly high values. This can be diagnosed by comparing results obtained over different flow rate ranges, and the accuracy can be improved by use of a narrow range of flow rates. The electrochemical method can be routinely applied during measurement of a polarization curve, without any addition equipment, and is well suited for rapid assessment of the effects of operating conditions and changes over time. (C) The Author(s) 2018. Published by ECS.