A passive vapor-feed direct methanol fuel cell (DMFC), which has a membrane vaporizer and a hydrophobic vapor transport layer (VTL), was investigated experimentally to improve its performance when highly concentrated or neat methanol was directly used. DMFCs with different structure parameters, such as the VTL thickness, open area ratio of the vaporizer, and thickness of the water management layer, were tested using different methanol concentrations varying from 6 M to neat methanol. The results showed that the performance of the passive vapor-feed DMFC mainly depended on the trade-off between the methanol vapor supply to the anode and methanol crossover through the membrane. Thickening the VTL, or decreasing the open area ratio of the vaporizer, effectively decreased the transport rate of methanol vapor to the anode, leading to an increase in the concentration of the optimum feeding methanol. The hydrophobic water management layer in the cathode was also useful in lowering the methanol crossover and increasing the membrane hydration level. Through optimizing the structure parameters, a passive vapor-feed DMFC fed with neat methanol, which showed a peak power density of 34 mW cm(-2) and a high fuel efficiency of 62% at room temperature, was developed. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3435256] All rights reserved.