Independent transport measurements of carbon dioxide permeation were undertaken to evaluate a standard assumption of attributing cathodic CO2 flux in direct methanol fuel cells (DMFCs) entirely to methanol crossover. With a humid air cathode at ambient conditions, a DMFC cathode emanated 3.5 to 4 x 10(-8) mol/(cm(2) s) of CO2 (20 to 25 mA/cm(2) "leak current density''). From a methanol-free anode feed of either carbonated liquid water or humid CO2/H-2 gas (90%: 10%), carbon dioxide permeated to the cathode at 0.7 x 10(-8) mol/(cm(2) s) or 20% of the total DMFC flux. Under current (enabled with the presence of H-2 in CO2), the permeation rate rose to 1 x 10(-8) mol/(cm(2) s). The rise with cell current density increase from 24 to 80 mA/cm(2) was quantitatively consistent with convection of CO2 with electro-osmosis of water. The indication that anodic CO2 contributes to the total cathodic flux has a positive implication for fuel efficiency in DMFCs. (C) 2004 The Electrochemical Society.