The stereospecific polymerization of isobutyl vinyl ether (IBVE) initiated with a catalyst system composed of AlEt2Cl and PhNH2, and the crystalline compound, (EtClAl)2NPh, which was separated from the above catalyst system, was studied. The compound, (EtClAl)zNPh, is dimeric in benzene and with diethyl ether and tetrahydrofuran forms 1 : 1 complexes which have no catalytic activity for the polymerization of IBVE. The stereospecificity of polymerization increased on increasing number of times the catalyst was recrystallized. In a highly purified polymerization system, this catalyst gave no polymer of IBVE, however the substances coexisting in the system, especially oxygen, were found to be effective cocatalyst for the stereospecific polymerization of IBVE. KEY WORDS Stereospecific Polymerization 1 Isobutyl Vinyl Ether 1 Aluminum Diethyl Cloride 1 Donor-Acceptor Complex 1 Cocatalyst I (EtClAl)zNPh Catalyst 1