Catalyst comprising (a) a source of chromium; (b) a ligand containing at least one phosphorus atom bound to at least one hydrocarbyl or heterohydrocarbyl group having a polar substituent, but excluding the case where all such polar substituents are phosphane, arsane or stibane groups, said ligand having the formula (R 1)(R Z)X-Y-X(R 3)(R 4) wherein X is phosphorus; Y is 1,2-phenylene, 1,3-propane, 1,2-catechol, 1,2-dimethylhydrazine, or -N(R 5)-where R 5 is hydrogen, hydrocarbyl or substituted hydrocarbyl. R 1, R 2, R 3 and R 4 are independently selected from methyl, ethyl, ethylenyl, propyl, butyl, cyclohexyl, benzyl, phenyl, tolyl, xylyl, mesityl, biphenyl, naphthyl, anthracenyl, methoxy, ethoxy, phenoxy (i.e. -OC 6 H 5), tolyloxy (i.e. -OC 6 H 4 (CH 3)), xylyloxy, mesityloxy, dimethylamino, diethylamino, methylethylamino, thiomethyl, thiophenyl, trimethylsilyl or dimethylhydrazyl, and wherein the polar substituents are independently selected from methoxy, ethoxy, isopropoxy, C 3 -C 20 alkoxy, phenoxy, pentafluorophenoxy, trimethylsiloxy, dimethylamino, methylsulphanyl, tosyl, methoxymethyl, methylthiomethyl, 1,3-oxazolyl, methoxymethoxy, hydroxyl, amino, sulphate, nitro, phosphane, arsane and stibane, and (c) an activator. (a) a source of chromium; (b) a ligand containing at least one phosphorus atom bound to at least one hydrocarbyl or heterohydrocarbyl group having a polar substituent, but excluding the case where all such polar substituents are phosphane, arsane or stibane groups, said ligand having the formula (R 1)(R Z)X-Y-X(R 3)(R 4) wherein X is phosphorus; Y is 1,2-phenylene, 1,3-propane, 1,2-catechol, 1,2-dimethylhydrazine, or -N(R 5)-where R 5 is hydrogen, hydrocarbyl or substituted hydrocarbyl. R 1, R 2, R 3 and R 4 are independently selected from methyl, ethyl, ethylenyl, propyl, butyl, cyclohexyl, benzyl, phenyl, tolyl, xylyl, mesityl, biphenyl, naphthyl, anthracenyl, methoxy, ethoxy, phenoxy (i.e. -OC 6 H 5), tolyloxy (i.e. -OC 6 H 4 (CH 3)), xylyloxy, mesityloxy, dimethylamino, diethylamino, methylethylamino, thiomethyl, thiophenyl, trimethylsilyl or dimethylhydrazyl, and wherein the polar substituents are independently selected from methoxy, ethoxy, isopropoxy, C 3 -C 20 alkoxy, phenoxy, pentafluorophenoxy, trimethylsiloxy, dimethylamino, methylsulphanyl, tosyl, methoxymethyl, methylthiomethyl, 1,3-oxazolyl, methoxymethoxy, hydroxyl, amino, sulphate, nitro, phosphane, arsane and stibane, and X is phosphorus; Y is 1,2-phenylene, 1,3-propane, 1,2-catechol, 1,2-dimethylhydrazine, or -N(R 5)-where R 5 is hydrogen, hydrocarbyl or substituted hydrocarbyl. R 1, R 2, R 3 and R 4 are independently selected from methyl, ethyl, ethylenyl, propyl, butyl, cyclohexyl, benzyl, phenyl, tolyl, xylyl, mesityl, biphenyl, naphthyl, anthracenyl, methoxy, ethoxy, phenoxy (i.e. -OC 6 H 5), tolyloxy (i.e. -OC 6 H 4 (CH 3)), xylyloxy, mesityloxy, dimethylamino, diethylamino, methylethylamino, thiomethyl, thiophenyl, trimethylsilyl or dimethylhydrazyl, and wherein the polar substituents are independently selected from methoxy, ethoxy, isopropoxy, C 3 -C 20 alkoxy, phenoxy, pentafluorophenoxy, trimethylsiloxy, dimethylamino, methylsulphanyl, tosyl, methoxymethyl, methylthiomethyl, 1,3-oxazolyl, methoxymethoxy, hydroxyl, amino, sulphate, nitro, phosphane, arsane and stibane, and (c) an activator.