Copolymerization of 2,3-dimethylmaleic anhydride and ethylene has been accomplished under ethylene pressure (up to 1000 psi) with AIBN as the initiator. The copolymers were obtained at relatively low yield and only of moderate molecular weight. The incorporation of 2,3-dimethylmaleic anhydride units into the copolymer is about 20 mol% at 1000 psi and is 33 mol% at 500 psi of ethylene pressure. Unlike maleic anhydride-ethylene copolymers, alternating 2,3-dimethylmaleic qnhydride-ethylene copolymers of reasonable molecular weight have not yet been prepared. 2,3-Dimethylmaleic anhydride-ethylene copolymers could be hydrolyzed to the polymeric acids and quantitatively esterified to the polymeric methyl esters. Both anhydride and ester copolymers have been characterized spectroscopically and by their thermal behavior.