A catalyst composition suitable for the trimerization of olefinic monomers, wherein the catalyst composition comprises: a) a source of chromium, molybdenum or tungsten; b) a ligand of general formula (I); (R.sup.1)(R.sup.2)P--X--P(R.sup.3)(R.sup.4) (I) wherein: X is a bivalent organic bridging group; R.sup.1 and R.sup.3 are independently selected from, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl and substituted heterohydrocarbyl groups, with the proviso that when R.sup.1 and R.sup.3 are cycloaromatic groups they do not contain a polar substituent at any of the ortho-positions; R.sup.2 and R.sup.4 are independently selected from optionally substituted cycloaromatic groups, each R.sup.2 and R.sup.4 bearing a polar substituent on at least one of the ortho-positions; and c) a cocatalyst. The present invention further relates to a process for the trimerization of olefinic monomers, particularly the trimerization of ethylene to 1-hexene, wherein the process comprises contacting at least one olefinic monomer with the catalyst composition described hereinabove.