The present invention relates to a polymerisation process, in particular to the polymerisation of olefins in a reactor system comprising two reactors in series, and most particularly provides a process for the polymerisation of monomer in at least first and second reactors operated in series, which process comprises contacting a first stream comprising vapour derived from the effluent withdrawn from the second reactor with a feed stream to the second reactor, said feed stream comprising effluent derived from the first reactor.