The in situ synthesis of methylalumoxane (MAO) on the crystalline structure of heteropoly acids generates a highly active and weakly coordinating compound for the metallocene catalyst. The MAO-phase formed by the controlled hydrolysis of trimethylaluminum covers the Keggin unit of the heteropoly compound, as is verified with several spectroscopic techniques. The primary Keggin structure of the heteropoly compound is preserved and MAO-formation induces an intensive charge transfer in the VIS-region. Al-27-NMR of alumoxo-phosphotungstate illustrates the reversible interaction of the Lewis acid Al of the anchored alumoxane with the bridging oxygen atoms of the phosphotungstate Keggin structure. The interaction of the alumoxo-heteropoly compound with an ansa-metallocene produces a highly active catalyst for the co-oligomerization of ethene and propene. The softness of the heteropoly compound combined with the pseudo-liquid phase formation of the alumoxo-heteropoly compounds exceeds the activity of the homogeneous system and the stability of the heterogeneous MAO-anchored materials. The composition of the Keggin structure determines the catalytic potential of the metallocene which is transformed in a temperature dependent oligomerization and a fluctuating alternation of monomer and comonomer. The molecular weight and the methyl branching of the oligomer molecules are designing tools for the physicochemical features of a synthetic lubricant. A comparable charge delocalization over each heteropoly compound results in a single-sited catalytic system inducing a small dispersity of oligomer products. (C)) 1999 Elsevier Science B.V. All rights reserved.