I-V characteristics were investigated for a direct type fuel cell with Pt-Ru catalyst, using acetaldehyde aqueous solution as a fuel. The cell voltage decreased from 0.26 to 0.19 V at 20 mA cm(-2) as the fuel concentration increased from 0.1 to 1 mol dm(-3). This result was compared with the data obtained with ethanol aqueous solutions. The cell voltage increased from 0.19 to 0.24 V at 20 mA cm-2 as the concentration of ethanol increased from 0.1 to 1.0 mol dm(-3). The anode and cathode overvoltages were measured using a dynamic hydrogen electrode as a reference electrode to investigate the origin of the voltage drop observed with acetaldehyde solutions. It was found that the opposite relation between the cell voltage and the fuel concentration was originated in the anode polarization. It is confirmed that the oxidation process of acetaldehyde to carbon dioxide with Pt-Ru catalyst is different from that of ethanol to acetaldehyde. (c) 2005 Elsevier B.V. All rights reserved.