The present study reports on experimental investigations of the dynamic behavior of CO2 gas bubbles and the performance of a 9 cm(2) transparent direct methanol fuel cell (DMFC). The movement of CO2 gas bubbles in the anode channel subjected to a flow of aqueous methanol solution was visualized. A series of parametric studies was carried out to evaluate the effects on the CO2 gas bubbles dynamics as well as the cell performance. It was observed that the pores around the comer of the channel ribs and the intersection of the carbon cloth fibres were favorable sites for the emergence of CO2 gas bubbles. The growth and coalescence of CO2 gas bubbles resulted in gas slugs blocking the channel and the pores in porous diffusion layer as well. Then the gas slugs were pushed by the aqueous methanol solution flow to detach and sweep downstream, clearing all the existing small bubbles on the porous diffusion layer surface. The processes of emergence, growth, coalescence, detachment, and sweeping of the gas bubbles were found to occur periodically. High flow rates of the aqueous methanol solution resulted in small discrete CO2 gas bubbles and short gas slugs. Increasing temperature of the methanol solution increased the quantity of CO2 gas bubbles. More CO2 gas bubbles and large gas slugs appeared in the channels with increasing pressure difference between the anode and the cathode. The cell performance was improved with increasing aqueous methanol flow rates, feed temperature, feed concentration, and the pressure difference between the anode and the cathode. (C) 2007 Published by Elsevier B.V.