The terpolymerization of ethyl methacrylate (EMA), N-phenylmaleimide (NPMI), and itaconic acid (IA) was investigated. The terpolymer composition was determined by elemental analysis and H-1 NMR spectroscopy. The reactivity ratios of the three binary systems (EMA/NPMI, EMA/IA, and NPMI/IA) were calculated and used for the calculation of the terpolymer composition with the terminal model equations. A comparison between the experimental and theoretical compositions was made. The rate of the terpolymerization process was measured dilatometrically at two total monomer concentrations; this was done to establish the presence of intermolecular interactions between the investigated monomers. The thermal analysis of the obtained terpolymers was performed by thermogravimetric analysis. (C) 2003 Wiley Periodicals, Inc.