Abstract The copolymerization of itaconic acid monomethylester (1‐carboxy‐1‐carbomethoxy‐methylethylene) with acrylonitrile in presence and in absence of ZnCI2 was investigated. The reactivity ratios of the monomer pairs itaconic acid monomethyl ester‐acrylonitrile (r1=0,05±0,02; r2=0,21±0,02) and itaconic acid monomethylester/ZnCl2‐acrylonitrile (r1=0,14±0,05; r2=0,41±0,03) are determined by the Kelen‐Tüdős and Fineman‐Ross method.