The effect of the ionomer content in the catalyst layers of both the anode and the cathode with the black catalyst on the performance of a passive DMFC was investigated in order to increase the power output of the passive DMFC. In situ cyclic voltammetry has been carried out to evaluate the electrochemically active surface area, ECSA and the catalyst utilization. Under the passive conditions, ionomer content had a significant effect on both the mass transport and the ECSA. The optimum ionomer content was affected by the operating current density whether at anode or at cathode. Under low current density region, i.e., activation over voltage region, 20 wt.% showed the highest performance at the anode and the cathode, and the cell performance in this region was varied in accordance with the results of the ECSA. Under high current density region, i.e., mass transport over voltage region, lower ionomer content, 10 and 15 wt.%, had the highest cell performance at the anode and the cathode, respectively. The decrease in the optimum ionomer content at high current density was related to the low mass transport of methanol at the anode, and the flooding at the cathode at the high ionomer contents. The optimum ionomer content whether at anode or at cathode was 20 wt.%, from the power density point of view. The catalyst utilization was 10% and 25% at optimum conditions at the cathode and the anode respectively. (C) 2010 Elsevier B.V. All rights reserved.