Catalyst compositions useful for olefin polymerizations comprising a Group 3-10 metal complex and a compound corresponding to the formula: AlArfQ1Q2, or a dimer, adduct, or mixture thereof and further mixtures with aluminum compounds of the formula AlArf3, where: Arf is a fluorinated aromatic hydrocarbyl moiety of from 6 to 30 carbon atoms; Q1 is Arf or a C1-20 hydrocarbyl group, optionally substituted with one or more cyclohydrocarbyl, hydrocarbyloxy, hydrocarbylsiloxy, hydrocarbylsilylamino, hydrocarbylsilyl, silylhydrocarbyl, di(hydrocarbylsilyl)amino, hydrocarbylamino, di(hydrocarbyl)amino, di(hydrocarbyl)phosphino, or hydrocarbylsulfido groups having from 1 to 20 atoms other than hydrogen, or, further optionally, such substituents may be covalently linked with each other to form one or more fused rings or ring systems; and Q2 is an aryloxy, arylsulfide or di(hydrocarbyl)amido group, optionally substituted with one or more hydrocarbyl, cyclohydrocarbyl, hydrocarbyloxy, hydrocarbylsiloxy, hydrocarbylsilylamino, hydrocarbylsilyl, silylhydrocarbyl, di(hydrocarbylsilyl)amino, hydrocarbylamino, di(hydrocarbyl)amino, di(hydrocarbyl)phosphino, or hydrocarbylsulfido groups having from 1 to 20 atoms other than hydrogen, or, further optionally such substituents may be covalently linked with each other to form one or more fused rings or ring systems, said Q2 having from 3 to 20 atoms other than hydrogen are useful as activators for olefin polymerizations.