A four-component homogeneous catalyst system comprising a substituted cyclopentadiene ligand, a chromium compound, an alkyl aluminium and a halogen source is effective for the trimerisation of ethylene to 1-hexene in good selectivity and activity. In a typical example a catalyst system containing chromium(III) tris (2-ethylhexanoate), 1,2,3,4,5-pentaphenylcyclopenta-1,3-diene, hexachloroethane and triethylaluminium produced 1-hexene in up to 76 mass%, at an activity of 40 700 g of product/(g Cr h). Approximately 15 mass per cent of internal hexenes are also produced as well as smaller quantities of C-10's and polyethylene. The effect of changes in temperature, pressure and reagent ratios has been investigated as well as the use of other substituted cyclopentadienes. (C) 2003 Elsevier B.V. All rights reserved.