The reactions of Cp(2)MtMeCl (Cp = eta(5)-C5H5, Mt = Ti, Zr, Me = CH3) with AlMe3 and with MeAl[OAl2H4](2)O, as a model of the cocatalytic site of methylalumoxane (MAO), were studied by means of DFT quantum-mechanical calculations. The energy required to form the Cp(2)MtMe(+) cation from dissociation of Cp(2)MtMeCl.AlMe3 and different kinds of chlorine-bridged and oxygen-bridged Cp(2)MtMeCl. MAO model adducts accounts for the higher cocatalytic activity exhibited by methylalumoxane with respect to AlMe3 in olefin polymerization. Both the presence of highly acidic aluminum atoms and negative charge dispersion power of XMAO(-) macroanions (X = Cl, Me) are essential features in determining low dissociation energies. Possible key roles played by monomer and AlMe3 content of MAO in affecting active species formation are suggested. The computational results support the idea that, in the absence of degradative side reactions, titanium and zirconium MAO-based systems should exhibit comparable catalytic activity.