A first route to the isomerisation of erthro- and threo-2,3-disubstituted succinyl units in alternating alkene-maleic acid copolymers is presented. The isomerisation reactions of the disodium salt of ethene-maleric acid copolymer were carried out in aqueous solution at 180-240degreesC in an autoclave. It is demonstrated that the erythro-content can be changed from 17% up to 67%. The dissociation of melting C-H bonds with the intermediate formation of a sp(2) hybridised carbon is proposed as the mechanism.