A catalyst precursor composition comprising a) a source of chromium, molybdenum or tungsten; b) a first ligand having the 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 aromatic 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 aromatic groups, each R.sup.2 and R.sup.4 bearing a polar substituent on at least one of the ortho-positions; and c) a second ligand having the general formula (II); (R.sup.1')(R.sup.2')P--X'--P(R.sup.3')(R.sup.4') (II) wherein X' is a bridging group of the formula --N(R.sup.5')--, wherein R.sup.5' is selected from hydrogen, a hydrocarbyl group, a substituted hydrocarbyl group, a heterohydrocarbyl group, a substituted heterohydrocarbyl group, a silyl group or derivative thereof; and R.sup.1', R.sup.2', R.sup.3' and R.sup.4' are independently selected from hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl and substituted heterohydrocarbyl groups of which at least one of R.sup.1', R.sup.2', R.sup.3' and R.sup.4' is substituted with a polar substituent, or R.sup.1', R.sup.2', R.sup.3' and R.sup.4' are independently selected from hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl and substituted heterohydrocarbyl groups, wherein any substituents on one or more of R.sup.1', R.sup.2', R.sup.3' and R.sup.4' are non-polar. The present invention also relates to a catalyst system comprising the catalyst precursor composition of the present invention and a cocatalyst.