Dependence of the rate constant of initiation on the overall concentration and composition of monomer has been investigated in the free radical copolymerization of ethyl acrylate and styrene in bulk and in benzene solution at 50° C. The rate constant of initiation has been determined by the inhibition method using triphenyl-verdazil as the stable free-radical inhibitor. An equation has been derived to calculate the rate constant of initiation in a copolymerization system, where both monomers undergo a pseudounimolecular side reaction with the inhibitor. The rate constant of initiation in the copolymerization mixture is a linear composition of rate constants determined separately in the pure constituents of the system.