Copolymers containing N-(4-acetyl phenyl)maleimide (NAPMI) and styrene (ST) at different compositions have been prepared by radical copolymerization. FTIR, H-1-, and C-13 NMR and elemental analysis techniques were used on the identification of polymers. The monomer reactivity ratios were estimated using the Kelen-Tudos (KT) and Fineman-Ross (FR) graphical methods. Structural parameters of the copolymers were obtained calculating the dyad monomer sequence fractions. A thermal study of the obtained copolymers related to the N-(4-acetyl phenyl)maleimide content has been carried out taking into account several theories predicting the depence of the glass transition temperature on the copolymers composition. The T(g)s are linearly distributed between T(g)s of homopolymers an in all cases, single, well-defined T(g)s are obtained. Glass transition temperatures increase with the increase of molar fraction of N-(4-acetylphenyl)maleimide. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 964-970, 2004.