Compositions comprising: A) an aluminum compound corresponding to the formula AlArf3, where Arf is a fluorinated aromatic hydrocarbyl moiety of from 6 to 30 carbon atoms; B) an aluminum compound corresponding to the formula: AlArfQ1Q2, or a dimer, adduct, or mixture thereof; where: Arf is as previously defined; 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; and the molar ratio of A):B) in the composition being from 0.1:1 to 10:1 are useful as activators for olefin polymerizations.