The invention relates to solid polyelectrolyte membranes and processes for their production. As solid polyelectrolyte materials, there have been known fluoropolymers which have perfluoroskeletons and side chains bearing sulfonic acid groups and heat-resistant resins which have hydrocarbon skeletons and contain alkylsulfonic acid or alkylphosphoric acid groups introduced thereto. The fluoropolymers are excellent in heat resistance and chemical resistance but have the problem of being expensive, while the heat-resistant resins have the problem of being poor in chemical resistance. Further, solid polyelectrolyte materials having higher electric conductivities have been desired. With the purpose of providing solid polyelectrolyte membranes having satisfactorily high electric conductivities and being excellent in heat resistance and chemical resistance and processes for producing the membranes, solid polyelectrolyte membranes are made from resins bearing both phosphoric acid groups and sulfonic acid groups which resins are obtained by copolymerizing a monomer bearing in the molecule a phosphoric acid group and an ethylenically unsaturated bond with a monomer bearing in the molecule a sulfonic acid group and an ethylenically unsaturated bond.