The ethanol electro-oxidation at gas diffusion electrodes made of different catalysts, Pt/C, PtRu(1:1)/C, and PtSn(7:3)/C, were studied by on-line differential electrochemical mass spectrometry in a wide temperature range (30-90 degrees C) as a function of the anode potential, the fuel concentration, and catalyst loading. The CO2 current efficiency (CCE) of the ethanol oxidation reaction (EOR) exhibits a maximum at about 0.6 V and decreases rapidly with further increasing potentials. The CCE for the EOR goes down with the increase in concentration of ethanol. CCE for ethanol oxidation reaction shows a strong increase with increasing catalyst loading. The CCE increases with increasing temperature, exceeding 75% at 90 degrees C, 0.1 M ethanol, and 5 mg/cm(2) Pt catalyst loading. PtSn/C shows high CCE, like Pt/C. But PtRu/C exhibits very small CCE. Of the intermediates, acetaldehyde is quite active for further oxidation. But acetic acid is fairly resistant against further oxidation. Our results indicate that the C-C bond scission observed for the EOR with CCE in excess of 50% has to proceed in parallel with ethanol oxidation to either acetaldehyde or acetic acid, and not sequentially from acetic acid further on, as acetic acid cannot be oxidized any further.