Experiments with independent pressurization of the direct methanol fuel cell anode and cathode allow for the observation of DMFC operation with carbon dioxide gas formation suppressed. Results indicate that the limiting current density is strongly related to the applied pressure, and, therefore, to the presence of CO(2) in the liquid phase. An additional experiment where CO(2) is allowed to accumulate in recycled anode fuel solution over a period of time and is then stripped from solution using nitrogen gas indicates that the presence of CO(2) in anode fuel solution at any pressure contributes to significant decreases in power and current density. Because CO(2) bubbles are ubiquitous in direct methanol fuel cells, this finding is key to the optimization of these systems. (C) 2010 Elsevier B.V. All rights reserved.