Abstract: The invention relates to a catalyst composition for olefin polymerization comprising metallocene complexes and novel cocatalysts belonging to the group of perfluorinated oligoarylaluminates, oligoarylthioaluminates and oligoarylaminoaluminates. These cocatalysts are produced by two alternative synthetic methods, either by reacting alkylalumoxanes with pentafluorophenol, pentafluorothiophenol, or with pentafluoroaniline, respectively; or in two-step processes comprising first reacting trialkylaluminum compounds with said perfluorinated agents and then by reacting the products of the first step with water. Catalyst systems containing these cocatalysts and metallocene complexes are active in polymerization and copolymerization reactions of ethylene and alpha-olefins.