A method for monitoring an industrial emulsion copolymerization of acrylonitrile and butadiene for the production of two NBR grades is presented. From an on-line calculation of the reaction heat (proportional to the mass flow rate of evaporated refrigerant), three open-loop estimators of increasing complexity were developed for monitoring conversion, copolymer composition, and average molecular weights. A good agreement between on-line estimates and off-line measurements was obtained.