A generalized copolymerization rate equation is proposed. The North equation of copolymerization is a special case that can be derived from the generalized equation. According to the mass balance and charge-transfer complex (CTC) equilibrium equation, the free monomer concentration is differentiated from the feed monomer concentration. The effects on the copolymerization rate (the equilibrium constant, total monomer concentration, homopropagation constants, and reactivity ratios) are quantitatively calculated and discussed. The generalized equation may be applied to not only the systems with the participation of CTC, such as styrene/N-phenylmaleimide (PMI), but also those without the participation of CTC, such as methyl methacrylate/PMI.