Products yields from a direct ethanol fuel cell (DEFC) have been measured in the normal operating mode with O-2 at the cathode, and with N-2/H-2 at the cathode. Apparent yields of both acetic acid and CO2 are significantly higher with O-2 at the cathode, and this has been attributed to crossover effects. It is shown that ethanol crossing through the membrane reacts with oxygen at the cathode to produce acetic acid, which then crosses to the anode. In contrast, the main role of CO2 produced from ethanol at the cathode appears to be to inhibit CO2 crossover from the anode. The effects of acetaldehyde crossover from the anode to the cathode have also been investigated, and it has been shown that loss of acetaldehyde in this way is very high at elevated temperatures. Recommendations are made for how best to measure product yields from a DEFC. (C) 2010 Elsevier Ltd. All rights reserved.