Mesoporous silica supported aluminum chloride catalysts have been prepared and used as heterogeneous initiators for the cationic polymerization of styrene. Faster reactions with an improved control over molecular weight were obtained compared to the homogeneous Lewis acid. The concentration of silica surface hydroxyl groups has proven to have a major influence on the initiation and transfer reaction rate and on the polymer molecular weight as they Act both as co-catalyst in the initiation process with the Lewis acid and as possible terminating agents. The importance of the support pore size and active center loading on the activity of the catalyst and on the polymer molecular weight have also been examined in detail. Studies on catalyst reuse are also reported. (C) 2002 Elsevier Science B.V. All rights reserved.