A process for the oligomerisation of ethylene to predominantly 1-hexene or 1-octene or mixtures of 1-hexene and 1-octene, the process including contacting ethylene with a catalyst under ethylene oligomerisation conditions, said catalyst comprising: i) a source of chromium; ii) a ligating compound of the formula R 1 R 2 P 1 XP 2 R 3 R 4 or a litigating compound of the formula wherein P 1 and P 2 are phosphorus atoms; X is -N(R 9)-, where R 9 is an alkyl, cycloalkyl or aryl group; and R 1 to R 4 all are selected from the group consisting of aromatic or heteroaromatic moieties directly bonded to P 1 or P 2; and iii) optionally a catalyst activator or combination of catalyst activators; the process being characterized in that at least one of R 1, R 2, R 3, and R 4 is a substituted aromatic ring directly bonded to P 1 or P 2 and which is substituted at a ring atom adjacent to the ring atom bonded to P 1 or P 2 with a substituent Y, where Y is represented as: such that A is O or S; and R EWG is an electron withdrawing group, wherein R EWG is selected from a group consisting of fluoromethyl; difluoromethyl; trifluoromethyl; 1,1'-difluoroethyl; chlorodifluoromethyl; pentafluoroethyl; heptafluoropropyl; and nonafluorobutyl wherein the Y-substituted aromatic rings are phenyl groups, in which the Y substituent will be at the ortho position relative to the carbon ring atom bonded to P 1 or P 2. i) a source of chromium; ii) a ligating compound of the formula R 1 R 2 P 1 XP 2 R 3 R 4 or a litigating compound of the formula wherein P 1 and P 2 are phosphorus atoms; X is -N(R 9)-, where R 9 is an alkyl, cycloalkyl or aryl group; and R 1 to R 4 all are selected from the group consisting of aromatic or heteroaromatic moieties directly bonded to P 1 or P 2; and or a litigating compound of the formula wherein P 1 and P 2 are phosphorus atoms; X is -N(R 9)-, where R 9 is an alkyl, cycloalkyl or aryl group; and R 1 to R 4 all are selected from the group consisting of aromatic or heteroaromatic moieties directly bonded to P 1 or P 2; and iii) optionally a catalyst activator or combination of catalyst activators; the process being characterized in that at least one of R 1, R 2, R 3, and R 4 is a substituted aromatic ring directly bonded to P 1 or P 2 and which is substituted at a ring atom adjacent to the ring atom bonded to P 1 or P 2 with a substituent Y, where Y is represented as: such that A is O or S; and R EWG is an electron withdrawing group, wherein R EWG is selected from a group consisting of fluoromethyl; difluoromethyl; trifluoromethyl; 1,1'-difluoroethyl; chlorodifluoromethyl; pentafluoroethyl; heptafluoropropyl; and nonafluorobutyl wherein the Y-substituted aromatic rings are phenyl groups, in which the Y substituent will be at the ortho position relative to the carbon ring atom bonded to P 1 or P 2. the process being characterized in that at least one of R 1, R 2, R 3, and R 4 is a substituted aromatic ring directly bonded to P 1 or P 2 and which is substituted at a ring atom adjacent to the ring atom bonded to P 1 or P 2 with a substituent Y, where Y is represented as: such that A is O or S; and R EWG is an electron withdrawing group, wherein R EWG is selected from a group consisting of fluoromethyl; difluoromethyl; trifluoromethyl; 1,1'-difluoroethyl; chlorodifluoromethyl; pentafluoroethyl; heptafluoropropyl; and nonafluorobutyl wherein the Y-substituted aromatic rings are phenyl groups, in which the Y substituent will be at the ortho position relative to the carbon ring atom bonded to P 1 or P 2.