Of the trace anions observed, [4,3-OH] -and [6,4-OH] -possibly contain OH groups. These anions are consistently more intense than the corresponding anions without OH groups, for instance, [6,4-OH] -is always more intense than [6,4] -(Table ). These hydroxylated anions are not seen in commercial h-MAO (nor h-MAO synthesized on a preparative scale ). The persistence of these anions during in situ monitoring experiments suggests that their precursors are also persistent. This would only be possible when there is a deficit of Me3Al in the solution. The bimolecular reaction between MAO OH groups and monomeric Me3Al is diffusion-controlled with a low barrier. Earlier, we suggested that these hydrolyses proceed in poorly dispersed aqueous suspensions, wherein much Me3Al is incorporated into an insoluble gel, regardless of the initial stoichiometry.