The kinetics of isobutylene (IB) polymerization, initiated by 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4 initiator/coinitiator system in methyl chloride/methylcyclohexane solvent mixture at -90-degrees-C, was investigated. The existence of a dynamic equilibrium between dormant and active species was verified by obtaining ＇living＇ conditions and 100 % initiator efficiency using a large excess of initiator over coinitiator [TMPCl]0 >> [TiCl]4. It was shown that the polymerization rate is first order in monomer and is directly proportional to [TiCl4]0 and [TMPCl]0. From polymerization rate data the overall polymerization rate constant k(p)＇ = k(p)K1 was calculated, where kp is the rate constant of propagation an K1 - k1/k-1 is the equilibrium constant for the dormant-active species equilibrium. The rate of TMPCl consumption was shown to be first order in [TMPCl]. While polymer concentration increased during the rather slow initiator consumption, the polymerization rate did not accelerate as the concentration of active species, determined by the dormant-active species equilibrium, remained constant. From initiator consumption data k1 and k(p)/k-1 values were estimated. The estimated rate constant values were used to simulate monomer and initiator conversion histories versus time. The simulated histories were in good agreement with measured data.