Copolymerizations of 3-vinyl-1,4-butyrolactone (1) and acrylonitrile (2) initiated by dibenzoyl peroxide or 2,2'-azoisobutyronitrile were carried out. The following reactivity ratios were found for the bulk copolymerization at 70°C for 1 : rl =0,078 +0,037 and for 2: r2 =4,7f0,3. In addition the values Q1=0,033 and el=0,053 were obtained. The ring opening and the ring closing of the lactone moiety in the polymer with an alkaline and an acid were found to be reversible. ~