DFT (density-functional theory) calculations were performed to investigate the thermodynamics of formation of Olefin Separated Ion Pairs (OSIP) Cp(2)MtCH(3)(+)/C2H4/Cl2Al[O(AlMe3)AlHMe](2)(-) (Cp = eta(5)-C5H5, Mt = Ti, Zr, Me = CH3) from ethylene and Cp(2)MtMe.Cl2Al[O(AlMe3)AlHMe](2), a model of adduct produced by metallocene/methylaluminoxane (MAO) systems for olefin polymerization. The results account for the high cocatalytic activity of MAO and show that titanium complexes are potentially more active than zirconium homologues, as confirmed by low temperature polymerization tests.