It is known that a small amount of intermediate products, i.e., formaldehyde, formic acid and so on, is exhausted from a direct methanol fuel cell (DMFC). The production rates of such intermediates are affected by the methanol and water concentrations at the anode, and also the distribution of these products is variable. We investigated the production of the intermediates from a passive DMFC containing a porous carbon plate (PCP), which allows the use of methanol at high concentrations up to 100% due to the high resistivity to the methanol transport through the PCR The production rates of each intermediate and their distribution were measured not only for a DMFC employing various PCPs with different transportation resistivities, but also for a DMFC without PCR The results were analyzed in terms of the rate of methanol crossover (MCO) and water flux through the membrane. The detected intermediates were formaldehyde. formic acid, and methylformate, in accordance with previous reports. The production rates of the intermediates were strongly dependent on the flux of the MCO rather than the apparent methanol concentration. When the DMFC was operated under the rate limiting conditions of the methanol transport by the PCP, the production rates of the intermediates were low. However, when it was operated outside of the rate limiting conditions, the production rate increased with the increasing rate of methanol crossover. (c) 2008 Elsevier B.V. All rights reserved.