A process involves preparing an aqueous solution of a diolefin-maleic anhydride copolymer prepared by solution polymerization, said aqueous solution being free of tetramethylsuccinonitrile (TMSN), a by-product resulting from the decomposition of azobisisobutyronitrile (AIBN) used as a polymerization initiator for the copolymerization of the diolefin-maleic anhydride copolymer. The aqueous solution free of TMNS is obtained without the separation of the copolymer from the polymerization liquid in which the copolymer was prepared. The process is composed of the two distillation steps, the first distillation being for the removal of a solvent for polymerization and the second distillation being for the removal of water as well as TMSN. The first and second distillation steps can be carried out in a continuous or discontinuous manner. The first distillation step is carried out after and/or during the addition of water or a basic aqueous solution, and the second distillation can be effected with or without the addition of such liquid. This process is particularly effective for removal of TMSN which is highly toxic and causes problems from the safety point of view.