The composition of methylalumoxane (MAO) and its interaction with trimethylaluminum (TMA) have been investigated by a combination of chemical, spectroscopic, neutron scattering and computational methods. The interactions of MAO with donor molecules such as THF, pyridine and PPh3 as a means of quantifying the content of “free” and “bound” TMA have been evaluated, as well as the ability of MAO to produce [Me2AlL2]+ cations, a measure of the electrophilic component likely to be involved in the activation of single-site catalysts. THF, pyridine and diphenylphosphinopropane (dppp) give the corresponding TMA-donor ligand complexes accompanied by the formation of [Me2AlL2]+ cations. The results suggest that MAO contains not only Lewis acid sites but also structures capable of acting as sources of [AlMe2]+ cations. Another unique, but still unresolved, structural aspect of MAO is the nature of “bound” and “free” TMA. The addition of the donors OPPh3, PMe3 or PCy3 leads to the precipitation of polymeric MAO and shows that about that 1/4 of the total TMA content is bound to the MAO polymers. This conclusion was independently confirmed by pulsed field gradient spin echo (PFG-SE) NMR measurements which show fast and slow diffusion processes resulting from free and MAO-bound TMA, respectively. The hydrodynamic radius Rh of polymeric MAO in toluene solutions was found to be 12±0.3 Å, leading to an estimates for the average size of MAO polymers with about 50-60 Al atoms. Small angle neutron scattering (SANS) resulted in a radius RS = 12.0 ± 0.3 Å for MAO polymer, in excellent agreement with PFG-SE NMR experiments, a molecular weight of 1800 ± 100 g/mol and about 30 Al atoms per MAO polymer. The MAO structures capable of releasing [AlMe2]+ on reaction with a base were studied by quantum chemical calculations on MAO models (OAlMe)n(TMA)m for up to n = 8 and m = 5. Both –O−AlMe2−O− and –O−AlMe2−−Me− four-membered rings are about equally likely to lead to dissociation of [AlMe2]+ cations. The resulting MAO anions rearrange, with structures containing separated Al2O2 four-rings being particularly favourable. The results support the notion that catalyst activation by MAO can occur by both Lewis acidic cluster sites and [AlMe2]+ cation formation.