A process for the oligomerization of ethylene to predominantly 1-hexene or 1-octene or mixtures of 1-hexene and 1-octene includes contacting ethylene with a catalyst under ethylene oligomerization conditions. The catalyst comprises a source of chromium, a diphosphine ligating compound, and optionally an activator. The diphosphine ligating compound includes at least one optionally substituted fused cyclic structure including at least two rings, the optionally substituted fused cyclic structure including a 5- to 7-membered aromatic first ring bonded to a phosphorus atom, the aromatic first ring being fused to a 4- to 8-membered heterocyclic second ring, the heterocyclic second ring including a heteroatom which is separated by two ring atoms along the shortest connecting path from the phosphorous atom that is bonded to the first aromatic ring.