NOVELTY - A membrane/electrode assembly (10) for polymer electrolyte fuel cells, comprises an anode (13) and a cathode (14), each having a catalyst layer (11) containing a proton-conductive fluoropolymer (A1), and a polymer electrolyte membrane (15) containing a proton-conductive fluoropolymer (B1), disposed between the anode and the cathode. Each of the proton-conductive fluoropolymer (A1) and (B1) has an ion exchange capacity of 1.4-1.8 meq/g dry resin; and a water content of at most 150 mass%.    USE - For polymer electrolyte fuel cells useful as power sources for electric cars and for domestic use.    ADVANTAGE - In the polymer electrolyte fuel cell, power generation will not stop even when the power generation is initiated in such an environment that the temperature of the membrane/electrode assembly is at most 0 degrees C, without necessity to provide a device to prevent freezing. The proton-conductive polymer contained in the catalyst layer has a high ion exchange capacity of at least 1.4 meq/g dry resin, thus sufficient heat generation can be obtained by a reaction which takes place in the catalyst layer. By using the membrane/electrode assembly, it is possible to obtain polymer electrolyte fuel cell excellent in the zero degree starting performance. When the proton-conductive fluoropolymer (A1) has an ion exchange capacity of at most 1.8 meq/g dry resin, excessive swelling of the proton-conductive fluoropolymer (A1) can be suppressed, thus suppressing deterioration of the output voltage due to e.g. clogging of the catalyst layer or the gas diffusion layer. When the proton-conductive fluoropolymer (B1) has an ion exchange capacity of at most 1.8 meq/g dry resin, excessive swelling of the proton-conductive fluoropolymer (B1) can be suppressed, thus preventing deformation of the polymer electrolyte membrane, and peeling between the polymer electrolyte membrane and the catalyst layer due to the deformation.    DETAILED DESCRIPTION - A membrane/electrode assembly (10) for polymer electrolyte fuel cells, comprises an anode (13) and a cathode (14), each having a catalyst layer (11) containing a proton-conductive fluoropolymer (A1), and a polymer electrolyte membrane (15) containing a proton-conductive fluoropolymer (B1), disposed between the anode and the cathode. Each of the proton-conductive fluoropolymer (A1) and (B1) has an ion exchange capacity of 1.4-1.8 (preferably 1.5-1.7) meq/g dry resin; and a water content of at most 150 (preferably 65-130) mass%. Each of the proton-conductive fluoropolymer (A1) and (B1) is a perfluorocarbon polymer having ionic groups. An INDEPENDENT CLAIM is included for operating a polymer electrolyte fuel cell having a membrane/electrode assembly for polymer electrolyte fuel cells, involving initiating power generation in such an environment that the temperature of the membrane/electrode assembly for polymer electrolyte fuel cells is at most 0 degrees C.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of the membrane/electrode assembly.    Membrane/electrode assembly (10)    Catalyst layer (11)    Gas diffusion layer (12)    Anode (13)    Cathode (14)    Polymer electrolyte membrane. (15)