Compositions comprising: A) an aluminum compound corresponding to the formula AIArf3, where Arf is a fluorinated aromatic hydrocarbyl moiety of from 6 to 30 carbon atoms; B) an aluminum compound corresponding to the formula: AIArfQ1Q2, or a dimer, adduct, or mixture thereof; where: Arf is as previously defined; Q?1 is Arf¿ or C¿1-20? 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 Q?2¿ 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.