The controlled cationic polymerization of styrene using CumOH/AlCl3OBu2/Py initiating system in a mixture CH2Cl2/n-hexane 60/40 v/v at −40 and −60 °C is reported. The number-average molecular weights of the obtained polystyrenes increased with increasing monomer conversion (up to Mn = 85,000 g mol−1) although experimental values of Mn were higher than the theoretical ones at the beginning of the reaction that was ascribed to slow exchange between reversible-terminated and propagating species. The molecular weight distribution became narrower through the reaction and leveled of at the value of Mw/Mn = 1.8–2.0. A kinetic investigation revealed that the rate of polymerization was first-order in AlCl3OBu2 concentration meaning that monomeric counteranion (AlCl3OH− or AlCl) involved in the initiation and propagation steps of the reaction. It was also found that the rate of polymerization decreased with lowering temperature, which could be attributed to a decrease in concentration of free Lewis acid (AlCl3), the true coinitiator of polymerization, because of an increase in the tightness of its complex with dibutyl ether. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3736–3743, 2010