In the living cationic polymerization of isobutyl vinyl ether (IBVE) by the CH3CH(OiBu)OCOCH3 (1)/EtAlCl2 initiating system in the presence of the added base in hexane at +40-degrees-C, the stability of the initiating system 1/EtAlCl2, which form initiating species CH3CH+(OiBu) derived from 1, was investigated. In the presence of the Lewis base such as ethyl acetate or 1,4-dioxane, the active species was stable for 300 min even at +40-degrees-C in the absence of IBVE, and the living polymers were quantitatively obtained by adding IBVE. However, the active species was partly consumed by side reactions during the standing time for 60 min in the presence of a less basic additive such as ethyl benzoate, and about 50% of the active species was deactivated in the presence of methyl chloroacetate. Consequently, in the case of a less basic additive such as methyl chloroacetate (which was effective for the fast living polymerization), it can be seen that the careful selection of polymerization conditions was required. The living polymerization rate was dependent on the second order of EtAlCl2 concentration. EtAlCl2 induced the cleavage of 1 into CH3CH+(OiBu) and EtAl-Cl2(OCOCH3), and the reactivity of CH3CH+ (OiBu) and propagating carbocation may be controlled by EtAl-Cl2(OCOCH3) with the aid of other EtAlCl2. Et1.5AlCl1.5 exists as a bimetallic complex of EtAlCl2 and Et2AlCl, and it is expected that the polymers having a bimodal molecular weight distribution will be obtained due to two kinds of counteranions coming from EtAlCl2 and Et2AlCl. However, in the cationic polymerization of IBVE by 1/Et1.5AlCl1.5 in the presence of ethyl acetate, the living polymer exhibiting a unimodal and very narrow molecular weight distribution was obtained. Thereby, it was suggested that the counteranions, EtAl-Cl2(OCOCH3) and Et2Al-Cl(OCOCH3), exchange rapidly with each other.