Abstract: A polymer is disclosed which has a betaine structure on at least one side chain. The polymer according to this invention is suitable for use as a material for producing a pervaporation membrane. A process is described for producing the polymer according to this invention whereby betaine groups are introduced into a starting polymer. The betaine groups are introduced into the starting polymer by irradiation of the starting polymer with ionizing radiation and then treatment of the irradiated starting polymer with monomers that are capable of a graft reaction. Monomers that either already contain a betaine function or are capable of forming such a function are used. The betaine can be formed, e.g., by alkylating quaternation of a nitrogen atom present in the monomer if an acid group is introduced simultaneously with the alkylating agent.