Abstract: Ethylene is polymerised or copolymerised using a transition metal catalyst at a pressure in the range 300-1000 atmospheres and a temperature of at least 100.degree. C in the presence of a volatile inert diluent such as pentane, or at least one 1-olefine comonomer such as buten-1, or both. The catalyst can be a Ziegler type catalyst, an organo-compound of a transition metal such as a substituted silylmethyl, cyclopentadienyl, arene, norbornyl, .pi.-allyl or benzyl compound and may be supported on a suitable support such as silica, alumina, or magnesia. Other catalysts which may be used are supported chromium or molybdenum oxides. If a solid catalyst is used it is preferred to use a material which has been finely dispersed.