To provide a membrane/electrode assembly and a method for operating a polymer electrolyte fuel cell, whereby power generation will not be terminated even when the power generation is initiated in such an environment that the temperature of the membrane/electrode assembly is at most 0° C. A membrane/electrode assembly 10 for polymer electrolyte fuel cells, which comprises an anode 13 and a cathode 14 each having a catalyst layer 11 containing a proton-conductive fluoropolymer (A), and a polymer electrolyte membrane 15 containing a proton-conductive fluoropolymer (B), disposed between the anode 13 and the cathode 14, wherein each of the above proton-conductive fluoropolymer (A) and the above proton-conductive fluoropolymer (B) has an ion exchange capacity of from 1.4 to 1.8 meq/g dry resin, and each of the above proton-conductive fluoropolymer (A) and the above proton-conductive fluoropolymer (B) has a water content of at most 150 mass %.