Direct liquid methanol fuel cell tests were performed with membrane electrode assemblies ͑MEAs͒ fabricated with polyphosphazene-based proton-exchange membranes. The membranes were prepared from sulfonated poly͓bis͑3-methylphenoxy͒phosphazene͔ that was blended with polyacrylonitrile and then UV cross-linked using benzophenone as the photoinitiator. MEAs worked best when a high ion-exchange capacity ͑high conductivity͒ polyphosphazene membrane contacted the electrodes, in which case the fuel cell power output was nearly the same as that with Nafion 117 ͑for current densities р 0.15 A/cm 2 ͒, but the methanol crossover was three times lower than that of Nafion. With a three-membrane composite MEA ͑a methanol-blocking film sandwiched between two high conductivity membranes͒, there was a significant decrease in crossover ͑ten times lower than that of Nafion 117͒ with a modest decrease in current-voltage behavior.The polymeric membrane that separates the anode and cathode in a direct methanol fuel cell ͑DMFC͒ must exhibit a high protonic conductivity and low methanol crossover. While DuPont's Nafion is the membrane of choice for H 2 /air proton-exchange membrane fuel cells, methanol permeability in this polymer is unacceptably high for DMFC applications. Consequently, new membrane alternatives to Nafion have been proposed and tested, including sulfonated poly͑ether ketone͒, 1 sulfonated polysulfones and poly͑ether sulfones͒, 2 blends of Nafion and poly͑vinylidene fluoride͒, 3 and phosphoric acid doped polybenzimidazole. 4 In these and other studies, it was generally found that those membranes which blocked methanol permeation more effectively than Nafion had a proton conductivity substantially lower than Nafion ͑the exception being polybenzimidazole, but this membrane material can only be used in a high temperature fuel cell with a methanol vapor feed͒. The lower proton conductivity would necessitate the use of a thinner membrane in a fuel cell membrane electrode assembly ͑MEA͒ which would to some extent negate the improved barrier properties of the polymer.In this paper, we report for the first time on the performance of a polyphosphazene-based ion-exchange membrane in a direct liquid methanol fuel cell. The membranes were composed of sulfonated poly͓bis͑3-methylphenoxy͒phosphazene͔ ͑SPOP͒ which was blended with polyacrylonitrile ͑PAN, a tough, uncharged, and hydrophobic polymer͒ and then UV cross-linked. In previous studies we have shown that membranes composed only of sulfonated and UV cross-linked poly͓bis͑3-methylphenoxy͒phosphazene͔ possessed a high proton conductivity and very low methanol diffusion coefficient. 5,6 More recently, we have found that the addition of polyacrylonitrile to such membranes is advantageous because (i) the mechanical properties of the blended film are improved, (ii) the presence of polyacrylonitrile makes hot-pressing electrodes easier during MEA fabrication, and (iii) membranes with a substantial polyacrylonitrile content have a low methanol crossover because this polymer restricts swe...