Methylaluminoxane (MAO) is the most widely used activator for zirconocene-catalyzed olefin polymerization. Its basic mechanism of action is depicted in Scheme 1, illustrated using a particularly well-studied, ansa-metallocene complex rac-Me2Si(η 5 -C9H6)2ZrMe2 (SBIZrMe2); in addition to propene polymerization studies involving both MAO and other activators, NMR spectroscopic techniques have been used to study the activation process, as well as UV-Vis spectroscopy in this specific case. The latter technique shows that at least two contact ion-pairs (CIP) 1 and 2 are formed in varying amounts as a function of the Al:Zr ratio used for catalyst activation. One of these is more prone to form the hetero-dinuclear, outer-sphere ion-pair (OSIP 3), which is the dominant species present at high Al:Zr ratios. It is known that OSIP 3 and its polymeric analogues are resting states in hexene polymerization using this catalyst. More recently, the activation of metallocene complexes L2ZrX2 {L2 = Cp2, n-BuCp2, rac-(CH2)2Ind2 (rac-EBI), X = Cl or Me} by MAO using the technique of ESI-MS in fluorobenzene solvent has been studied. This technique, which selectively detects the cations and anions corresponding to OSIP 3, complements earlier spectroscopic studies in that activation of ansametallocene complexes by MAO is uncomplicated by formation of homodinuclear [(L2ZrMe)2-μ-X] + (X = Me, Cl) complexes. Unfortunately, ESI-MS does not provide information about CIP, as the [L2ZrMe] + cation is only indirectly detected through gas phase, collision induced dissociation of the [L2ZrMe2AlMe2] + cation. However, ESI-MS does provide the m/z ratio of the counteranions that are present. In the case of hydrolytic MAO from W. R. Grace (10 or 30 wt%) a remarkably simple negative ionspectrum is obtained using metallocene or other Lewis base donors such as octamethyltrisiloxane (OMTS). The dominant anion present has m/z 1375 and a likely composition based on MS/MS studies is [(MeAlO)16(Me3Al)6Me] -(hereinafter abbreviated [16,6] -).