A process for the tetramerisation of ethylene, 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 PXY wherein P is a phosphorous atom; X is a linking group between P and Y, wherein X is selected from dimethylmethylene, ethane-1,2-diyl, ethene-1,2-diyl, propane-1,2-diyl, propane-1,3-diyl, butane-2,3-diyl, -N(R 5 )-, -N(R 5 )-N(R 6 )-, =C(R 7 )-N(R 5 )-, or -N(R 5 )-C(R 7 )(R 8 )-, where R 5 and R 6 are independently a hydrocarbyl group, an organoheteryl group or a heterohydrocarbyl group, R 7 and R 8 are independently a hydrogen, a hydrocarbyl group, an organoheteryl group or a heterohydrocarbyl group; R 1 and R 2 are both independently aromatic, including heteroaromatic, groups directly bonded to P; and Y is an optionally substituted group that is represented as such that P is a phosphorous atom and bonds to X; and L 1 and L 2 are linkers selected from the group consisting of a covalent bond, -O-, -S-, -NR 3 -, -CR 3 R 4 -, -C(=O)- where R 3 and R 4 are independently a hydrogen, a hydrocarbyl group, a heterohydrocarbyl group or an organoheteryl group; and iii) optionally a catalyst activator or combination of catalyst activators. i) a source of chromium; ii) a ligating compound of the formula R 1 R 2 PXY wherein P is a phosphorous atom; X is a linking group between P and Y, wherein X is selected from dimethylmethylene, ethane-1,2-diyl, ethene-1,2-diyl, propane-1,2-diyl, propane-1,3-diyl, butane-2,3-diyl, -N(R 5 )-, -N(R 5 )-N(R 6 )-, =C(R 7 )-N(R 5 )-, or -N(R 5 )-C(R 7 )(R 8 )-, where R 5 and R 6 are independently a hydrocarbyl group, an organoheteryl group or a heterohydrocarbyl group, R 7 and R 8 are independently a hydrogen, a hydrocarbyl group, an organoheteryl group or a heterohydrocarbyl group; R 1 and R 2 are both independently aromatic, including heteroaromatic, groups directly bonded to P; and Y is an optionally substituted group that is represented as such that P is a phosphorous atom and bonds to X; and L 1 and L 2 are linkers selected from the group consisting of a covalent bond, -O-, -S-, -NR 3 -, -CR 3 R 4 -, -C(=O)- where R 3 and R 4 are independently a hydrogen, a hydrocarbyl group, a heterohydrocarbyl group or an organoheteryl group; and wherein P is a phosphorous atom; X is a linking group between P and Y, wherein X is selected from dimethylmethylene, ethane-1,2-diyl, ethene-1,2-diyl, propane-1,2-diyl, propane-1,3-diyl, butane-2,3-diyl, -N(R 5 )-, -N(R 5 )-N(R 6 )-, =C(R 7 )-N(R 5 )-, or -N(R 5 )-C(R 7 )(R 8 )-, where R 5 and R 6 are independently a hydrocarbyl group, an organoheteryl group or a heterohydrocarbyl group, R 7 and R 8 are independently a hydrogen, a hydrocarbyl group, an organoheteryl group or a heterohydrocarbyl group; R 1 and R 2 are both independently aromatic, including heteroaromatic, groups directly bonded to P; and Y is an optionally substituted group that is represented as such that P is a phosphorous atom and bonds to X; and L 1 and L 2 are linkers selected from the group consisting of a covalent bond, -O-, -S-, -NR 3 -, -CR 3 R 4 -, -C(=O)- where R 3 and R 4 are independently a hydrogen, a hydrocarbyl group, a heterohydrocarbyl group or an organoheteryl group; and such that P is a phosphorous atom and bonds to X; and L 1 and L 2 are linkers selected from the group consisting of a covalent bond, -O-, -S-, -NR 3 -, -CR 3 R 4 -, -C(=O)- where R 3 and R 4 are independently a hydrogen, a hydrocarbyl group, a heterohydrocarbyl group or an organoheteryl group; and iii) optionally a catalyst activator or combination of catalyst activators. <EOS>