The effect of ligand of Lewis acid on the cationic polymerization of isobutylene (IB) using TiCl4/TiBr4-mixed co-initiator is reported. The polymerization proceeds in a living fashion; however, the polymerization rate decreases with decreasing TiCl4/TiBr4 ratio. The active co-initiator in this polymerization system is the mixed dimer, Ti2ClxBr(8-x), not the dimer of TiCl4 or TiBr4. From the molecular k(i)(app) weight distribution vs conversion plots, the rate constant of deactivation, k(-i), and ionization, k(i), the apparent equilibrium constant of ionization, k(app), and the thermodynamic parameters are calculated and discussed. By the stepwise replacement of Cl to Br the Lewis acidity decreases, which results in a decreased ionization rate constant while the deactivation rate constant remains essentially unchanged. Therefore, the decreasing overall rates of polymerization with decreasing TiCl4/TiBr4 ratio are exclusively due to the lower equilibrium constant of ionization, which results in lower ion pair concentration.