The copolymerization of styrene (St, M(1)) and N-phenylmaleimide (PMI, M(2)) in chloroform using 2,2’-azobis(isobutyronitrile) as an initiator was investigated. A new kinetic model with assumption of homopolymerization and cross-propagation and diffusion-controlled termination was proposed. The reactivity ratios of free monomers and the charge-transfer complex (CTC) in the copolymerization were determined by a new method, and the values were obtained to be r(12) = 0.03370, r(21) = 0.01157, r(1C) = 0.002972, and r(2C) = 0.003379, respectively. A kinetic treatment based on the model of mechanism was used to quantitatively estimate the ratio of the CTC rate to the total copolymerization rate in the St/PMI copolymerization. It was about 33.0-55.0% in a wide range of monomer feed.