To provide an electrolyte polymer for fuel cells, an electrolyte membrane, a membrane/electrode assembly for fuel cells excellent in the durability. An electrolyte polymer for fuel cells made of a perfluorocarbon polymer having ion exchange groups (which may contain etheric oxygen atoms), characterized in that the value calculated by dividing an absorption area S.sub.CH derived mainly from a C--H bond in the range of from 3,100 cm.sup.-1 to 2,800 cm.sup.-1 by an absorption area S.sub.CF derived mainly from a C--F bond in the range of from 2,700 cm.sup.-1 to 2,000 cm.sup.-1, as measured by means of infrared spectrophotometry, is less than 0.005, an electrolyte membrane and a membrane/electrode assembly.