This invention relates to the oligomerisation of olefinic compounds in the presence of an oligomerisation catalyst activated in two stages by two catalyst activators According to the invention there is provided a process for activating an oligomerisation catalyst by contacting the catalyst with i) a first activator component selected from the group consisting of the aluminoxanes and a mixture of at least one aluminoxane and at least one organylaluminium compound, and ii) a second activator component which is an organylaluminium compound, the process being characterised therein that the oligomerisation catalyst is first contacted with one of the first activator component or second activator component, and the resulting mixture is thereafter contacted with the other of the first activator component or second activator component.