Utilizing highly concentrated methanol solution in the direct methanol fuel cells (DMFCs) is strongly preferable because it distinctly diminishes the cells size and simultaneously improves the power density. In this regard, water back-diffusion from cathode to anode is an important process as water is a reactant in the anode reactions. Beside the low cell efficiency, lack of water at the anode leads to form toxic intermediates which environmentally constrain the commercial applications. This study investigates exploiting hydrophobic cathode air filter as simple and effective strategy to enhance water back-diffusion. Typically, a passive vapor-feed DMFC using a porous carbon plate (PCP) on the anode surface was operated with different methanol concentrations up to 100% with and without the use of a hydrophobic cathode air filter. In situ mass spectrometry using a capillary probe was conducted for the anode gas layer to investigate the effect of the addition of the filter on the formation of reaction products at the anode surface. In normal DMFC, analysis of anode gas-layer indicated that formation methylformate (HCOOCH3) significantly increases upon utilizing high methanol concentrations. However, addition of the cathode air filter led to distinct decrease of the methanol crossover and increase of the water back-diffusion from the cathode to the anode especially at high methanol concentrations. The in situ gas analysis of the cell operated at 100 wt% (24.7 M) methanol revealed that the addition of the cathode filter significantly decreased the formation of the undesirable intermediates; 65% reduction in the amount of formed methylforrnate was observed. This effect was related to the increase in the ratio between the methanol and water vapor pressure (P-CH3OH/P-H2O) after addition of the filter and hence increased the performance of the passive DMFC when highly concentrated methanol was used as a fuel. (C) 2013 Elsevier B.V. All rights reserved.