A process for the preparation of polyolefins having a bi- or multimodal molecular weight distribution which comprises: (i) contacting olefin monomer and a first co-reactant with a catalyst system in a first reaction zone under first polymerisation conditions to produce a product comprising a first polyolefin having a first molecular weight and distribution; and (ii) contacting olefin monomer and a second co-reactant with a catalyst system in a second reaction zone under second polymerisation conditions to produce a second polyolefin having a second molecular weight distribution different from the first molecular weight distribution; wherein the first and second polyolefins are mixed together, wherein one of the co-reactants is hydrogen and the other is a comonomer selected from butene, methylpentene, hexene or octene, and wherein each catalyst system comprises (a) a metallocene catalyst component comprising a bis tetrahydroindenyl compound of the general formula (IndH4)2R''MQ2 in which each Ind is the same or different and is indenyl or substituted indenyl, R'' is a bridge which comprises a C1-C4 alkylene radical, a dialkyl germanium or silicon or siloxane, or an alkyl phosphine or amine radical, which bridge is substituted or unsubstituted, M is a Group IV metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; and (b) a cocatalyst which activates the catalyst component.