The cationic polymerization of isobutylene with H2O/ⁱBu2AlCl and H2O/ⁱBuAlCl2·nOR2 (n = 0-1; R2O = Bu2O, Hex2O, ⁱPr2O) initiating systems in toluene as a solvent at -20 °C has been investigated. The H2O/ⁱBu2AlCl initiating system induced slow cationic polymerization of isobutylene to afford polyisobutylenes with high molecular weight (up to Mn = 55 000 g mol⁻¹) with relatively low polydispersity (Mw/Mn < 2.5) and high exo-olefin end group content (>85%). The introduction of additional water into the system allowed increasing the reaction rate, but almost did not influence the molecular weight and exo-olefin content. The use of stronger Lewis acid ⁱBuAlCl2 results in a significant increase of the intensity of side reactions such as chain transfer to solvent (toluene) and isomerization of the growing macrocations, leading to the formation of ill-defined products. However, the addition of 0.6-1.0 equivalents of ethers to Lewis acid allowed conducting the polymerization in a controlled fashion in terms of chain end functionality. In addition, the molecular weight can be efficiently controlled by either the ether/Lewis acid ratio or the nature of the electron donor additive. Based on the obtained results, the polymerization mechanism, the key features of which are complex formation between Lewis acid and toluene and possible stabilization of active species through their interaction with toluene, was proposed.