Free-radical-initiated copolymerization of N-phenylmaleimide (NPMI) with styrene (St) at 110 degreesC in a toluene solution initiated by AIBN was carried out by a semibatch method. The compositions of the copolymers were determined by using an elemental analyzer. The glass-transition temperatures of the copolymers were measured by differential scanning calorimetry. All the semibatch copolymers show a single glass-transition temperature that increases markedly with increasing NPMI content in the copolymers. The thermal stabilities of the copolymers were studied by using a programmed thermogravimetric analysis technique. Copolymers show a considerable increase in thermal stability and different degradation reaction mechanisms with increasing content of NPMI.