A process for the oligomerization, preferably the tetramerization, 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 substituted aromatic ring bonded to a phosphorous atom. The substituted aromatic ring is substituted at a ring atom adjacent to the ring atom bonded to the respective phosphorous atom with a group Y, where Y is of the form —AREWG, A being O, S or NR5, where R5 is a hydrocarbyl, heterohydrocarbyl or organoheteryl group, and REWG is an electron withdrawing group.