Miscible Nafion((R))/poly(vinylidene fluoride) blend membranes are prepared and characterized PVdF = poly(vinylidene fluoride). The membranes have high miscibility above a 60 wt.% Nafion((R)) fraction when both polymers are dissolved in a solvent mixture of N,N'-dimethylacetamide and 2-propanol. Despite the high miscibility, the extremely hydrophobic PVdF component reduces the water uptake of Nafion((R))/PVdF blend, and the proton conductivity is much lower than that of a Nafion((R)) 115 membrane even with a high, weight fraction of Nafion((R)). To improve water affinity and proton conductivity, PVdF is chemically-modified by means of a dehydrofluorination reaction and H2SO4 doping prior to solution blend. The chemically-modified Nafion((R))/PVdF blend membrane shows similar conductance to Nafion((R)) 115 without any auxiliary fillers. Nafion((R))/PVdF blend membranes of similar to35 mum thickness are hot-pressed between catalyzed carbon paper ELAT((R)) electrodes. A 25 cm(2) single cell delivers a maximum power of about 440 mW cm(-2) at 900 mA cm(-2) under H-2/O-2, which is comparable to the performance of Nafion((R)) 115 under the same operating conditions. (C) 2003 Elsevier Science B.V. All rights reserved.