Mixed-feed direct methanol fuel cells, which operate on a single feed stream comprised of fuel and oxidant, were constructed using PtRu anode catalysts. Reaction selectivity favoring methanol oxidation at the anode was achieved by application of a barrier layer comprised of reconstituted Nafion, and optionally carbon black, hot pressed between the anode catalyst layer and reactant diffusion layer. The modified anode outperformed an unmodified anode in the presence of mixed feed at 60 and 80 degrees C. This result is attributed to reduced infiltration of oxygen from the mixed feed due to introduction of a barrier that admits mainly aqueous methanol. Inclusion of carbon black in the barrier layer led to similar to 30% increase in current density at 350 mV/dynamic hydrogen electrode, attributable to reduced contact resistance. On increasing the cell temperature from 60 to 80 degrees C, the activation polarization region vanished for anodes that were either unmodified or modified by a Nafion-carbon barrier layer. This result is attributed to decreased water content in the barrier at elevated temperature, leading to increased oxygen flux and thereby introducing a mixed potential at the anode. (C) 2008 The Electrochemical Society.