The anode products are observed when ethanol fuel is circulated in the direct ethanol fuel cell system using Nation((R)) products are M and acetaldehyde. I-V characteristics of a direct type fuel cell Using ethanol and acetaldehyde as fuels are investigated. Anode and cathode overpotentials are also measured to analyze the characters of the polarization Curves obtained for both fuels. The MEA consisted of PtRu anode catalyst. The voltage drops as the concentration of acetaldehyde Solution increases. In the case of ethanol solution, the voltage increases as the concentration increases. The anode overpotential incrcases as the concentration of acetaldehyde increases although the increase of cathode overpotential is smaller than that of anode overpotential. The opposite result is observed for ethanol Solutions, i.e., the anode overpotential increases as the concentration of ethanol decreases. This result shows that the voltage drop observed for acetaldehyde Solution results front the anode overpotential. Rotating disc electrode (RDE) measurements and polarization Curve measurements were also performed to confirrn the relation between acetaldehyde concentration and overpotentials. It is Supposed that the electrocatalytic oxidation mechanism of acetaldehyde oil PtRu catalyst is different from that of ethanol. (c) 2006 Elsevier Ltd. All rights reserved.