The structure and properties of methylaluminoxane (MAO) continue to attract interest in the context of olefin polymerization using metallocene or other single-site catalysts. After nearly 50 years of effort devoted to the study of this process, the identity of the reactive components of MAO has become clearer. Theory suggests that the bulk of this fluxional mixture consists of relatively unreactive, large cages or other structures built up of tetrahedral O3AlMe and trigonal O groups in six-membered, or larger (MeAlO)n rings. Reactive structures consist of strained cages, analogous to those adopted by the t-butylaluminoxanes, especially those with vicinal (MeAlO)2 rings, extended cages or nanotubes with reactive groups formed via the addition of structural Me3Al to the vertices and ends of these structures, and twodimensional sheet models with reactive sites incorporating structural Me3Al along their edges. Recently, a component of this complex mixture was finally isolated and characterized by X-ray crystallography. It has a formula of (MeAlO)26(Me3Al)9 (hereinafter 26,9) and a molecular weight of 2156 g mol , with a sheet structure and was used to activate metallocenes. The average molecular weight and presumably the molecular weight distribution of hydrolytic MAO (h-MAO) is known to be important in metallocene catalyst activation. Low MW species such as (Me2AlOAlMe2)n (n = 3 to 4) are known to be ineffective, while early synthetic methods emphasize the formation of higher MW material as confirmed through cryoscopic measurements. Cryoscopic measurements of commercial h-MAO (W. R. Grace) suggest a (number) average MW (𝑀 ̅ 𝑛 ) of ca. 1100 g mol while small angle neutron scattering (SANS) studies of h-MAO (Chemtura/Lanxess) suggest a higher (weight) average MW (𝑀 ̅ 𝑤 ) of 1803 ± 55 g mol with a spread in values of between 1400 -2800 g mol depending on free Me3Al content (𝑀 ̅ 𝑤 = 1803 corresponds to ca. 75% of total Me3Al content for the sample in question). The same study suggested a roughly spherical shape for bulk MAO based upon rg = 9.3 and rH =11.7 Å as measured by SANS and PGSE NMR, respectively. Both estimates were in good agreement with earlier studies of T1 relaxation (rg = 9.7 Å) and metallocene ion-pair size (rH = 12-12.5 Å) by PGSE NMR. However, interpretation of the PGSE NMR data in terms of aluminoxane molar mass (50-60 Al atoms for MAO20 vs. 150-200 Al atoms for the ion-pair22) differed significantly between the two studies. That the source and Me3Al content of MAO is an important variable is revealed by PGSE NMR studies on h-MAO (Sigma Aldrich), where dried or chemically modified MAO (DMAO and MAO-BHT), and the ion-pairs formed from Cp2ZrMe2 and DMAO had larger dimensions than the commercial material. [a] Prof. L. Rocchigiani . However, the relationship between hydrodynamic volume and molar mass is never straightforward.20 , 22 , 23