A fluorophenylborate useful as an activator for an olefin polymerization catalyst is represented by the formula: Ct.sup.+[B--(Ar.sup.fR.sub.n).sub.4].sup.- where Ct.sup.+ is a cation capable of extracting an alkyl group from, or breaking a carbon-metal bond of, an organo metallic compound; Ar.sup.f is a fluorophenyl group; n is 1 or 2; and each R is independently selected from a fluorophenyl group and a fluoronaphthyl group, provided that when n=1, each R group is connected at the 3-position relative the connection between the associated Ar.sup.f group and the boron atom and, when n=2; the R groups are connected at the 3-position and the 5-position respectively relative the connection between the associated Ar.sup.f group and the boron atom.