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 wherein P 1 and P 2 are phosphorus atoms; X consists of -N -, where R9 is an alkyl, cycloalkyl or aryl group; and wherein R 1 to R 4 all are selected from a group consisting of aromatic moieties directly bonded to P 1 or P 2; and iii) optionally a catalyst activator or combination of catalyst activators; the process being characterised therein that at least one of R 1, R 2, R 3, and R 4 of the ligating compound is represented as Z, where Z includes a fused bicyclic structure including an optionally substituted six-membered aromatic ring fused to an optionally substituted 4- to 8-membered heterocyclic second ring, and which is represented as: such that Y = O, S, N or NH; and L is a linking group between Y and the six-membered aromatic ring, wherein L is selected such that Z is an optionally substituted fused bicyclic heteroaryl group incorporating Y as a ring atom of this bicyclic heteroaryl group, where the ring including Y is a 5- or 6-membered ring, or, wherein L is selected such that Z is an optionally substituted fused bicyclic heteroaryl group including further fused ring structures to form a fused polycyclic structure with more than two rings, and wherein no more than two of R 1 to R 4 include a fused cyclic structure; and, optionally, wherein any one of R 1 to R 4 is independently linked to one of each other to form a cyclic structure. i) a source of chromium; ii) a ligating compound of the formula R 1 R 2 P 1 XP 2 R 3 R 4 wherein P 1 and P 2 are phosphorus atoms; X consists of -N -, where R9 is an alkyl, cycloalkyl or aryl group; and wherein R 1 to R 4 all are selected from a group consisting of aromatic moieties directly bonded to P 1 or P 2; and wherein P 1 and P 2 are phosphorus atoms; X consists of -N -, where R9 is an alkyl, cycloalkyl or aryl group; and wherein R 1 to R 4 all are selected from a group consisting of aromatic moieties directly bonded to P 1 or P 2; and iii) optionally a catalyst activator or combination of catalyst activators; the process being characterised therein that at least one of R 1, R 2, R 3, and R 4 of the ligating compound is represented as Z, where Z includes a fused bicyclic structure including an optionally substituted six-membered aromatic ring fused to an optionally substituted 4- to 8-membered heterocyclic second ring, and which is represented as: such that Y = O, S, N or NH; and L is a linking group between Y and the six-membered aromatic ring, wherein L is selected such that Z is an optionally substituted fused bicyclic heteroaryl group incorporating Y as a ring atom of this bicyclic heteroaryl group, where the ring including Y is a 5- or 6-membered ring, or, wherein L is selected such that Z is an optionally substituted fused bicyclic heteroaryl group including further fused ring structures to form a fused polycyclic structure with more than two rings, and wherein no more than two of R 1 to R 4 include a fused cyclic structure; and, optionally, wherein any one of R 1 to R 4 is independently linked to one of each other to form a cyclic structure. the process being characterised therein that at least one of R 1, R 2, R 3, and R 4 of the ligating compound is represented as Z, where Z includes a fused bicyclic structure including an optionally substituted six-membered aromatic ring fused to an optionally substituted 4- to 8-membered heterocyclic second ring, and which is represented as: such that Y = O, S, N or NH; and L is a linking group between Y and the six-membered aromatic ring, wherein L is selected such that Z is an optionally substituted fused bicyclic heteroaryl group incorporating Y as a ring atom of this bicyclic heteroaryl group, where the ring including Y is a 5- or 6-membered ring, or, wherein L is selected such that Z is an optionally substituted fused bicyclic heteroaryl group including further fused ring structures to form a fused polycyclic structure with more than two rings, and wherein no more than two of R 1 to R 4 include a fused cyclic structure; and, optionally, wherein any one of R 1 to R 4 is independently linked to one of each other to form a cyclic structure.