Abstract: There is disclosed a process for polymerizing a cationically polymerizable olefin comprising the step of polymerizing at least one cationically polymerizable olefin at a subatmospheric pressure in the presence of a cationic polymerization catalyst system which comprises an initiator and an activator, which together form a reactive cation and non-co-ordinating anion, the activator being prepared by the reaction of a metalloid compound of formula (R1R2R3)M with a co-initiator, the co-initiator being selected from the group consisting of an alcohol, a thiol, a carboxylic acid, a thiocarboxylic acid and the like.