A process (10) for polymerizing or oligomerising a hydrocarbon includes feeding at a low level a liquid hydrocarbon reactant (32) into a bulk liquid phase (14) comprising polymeric or oligomehc product admixed with a catalyst. The liquid hydrocarbon reactant is allowed to vapouhse to form bubbles rising through the bulk liquid phase (14) and to polymerise or oligomerise to form the polymeric or oligomeric product, with the rising bubbles creating turbulence in the bulk liquid phase (14), thereby mixing the bulk liquid phase (14). Gaseous components (26) comprising any unreacted vapouhsed hydrocarbon reactant and any gaseous product that may have formed are withdrawn from a head space above the bulk liquid phase (14). Liquid phase from the bulk liquid phase (14) is withdrawn (18) to maintain the bulk liquid phase (14) at a desired level.