The established model cluster (AlOMe)(16)(AlMe3)(6) for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethyl-aluminum; BHT = 2,6-di-test-butyl-4-methyl-phenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A-C to release neutral Al fragments (i.e., AlMe2 R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR]+) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B, albeit of lower Lewis acidity, should be still able to release cationic AI fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)](+) from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor- dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators.