This invention is directed to highly crosslinked, substantially water insoluble, cation exchange membranes prepared from homogeneous solutions comprising at least one substantially water soluble polar solvent (including water) at least one substantially water soluble polymerizable monomeric onium styrene sulfonate McKee type salt and/or substantially water soluble monomeric, polymerizable derivative thereof and at least one substantially water insoluble, di-, tri- or poly-ethylenic (vinyl or related) crosslinking monomer copolymerizable with said sulfonate salt. Membranes, especially useful in electrodialysis, may be obtained in "one-step" processes which require no further chemical reactions after polymerization.