By using sodium dodecyl sulfate (SDS) and pentanol (PTL) as emulsifiers, the oil-in-water microemulsion containing N-butyl maleimide (NBMI, M-1) and styrene (St, M-2) was prepared. The microemulsion copolymerization using potassium persulfate (KPS) as an initiator was investigated. On the basis of kinetic model proposed by SHAN Guo-Rong, the reactivity ratios of free monomers and the charge-transfer complex (CTC) in the copolymerization were found to be r(12) = 0.0420 r(21) = 0.0644 r(1C) = 0.00576 and r(2C) = 0.00785, respectively. A kinetic treatment based on this model was used to quantitatively estimate the contribution of CTC to the total copolymerization rate in the NBMI/St copolymerization It was about 17.0 - 20.0% for a wide range of monomer feed ratios.