Abstract: Polymethylaluminoxane compositions having increased solution stability in organic solvent and, in some cases, increased polymerization activity, which comprise moieties derived from an organic compound containing an electron-rich heteroatom (e.g., from Group V and/or Group VI of the Periodic Table) and a hydrocarbyl moiety. These compositions can be formed by reaction of trimethylaluminum, the organic compound, and water. They can also be formed by reaction of pre-formed polymethylaluminoxane the organic compound, which can be a hydrocarbyl group-containing compound containing an electron-rich heteroatom (e.g., oxygen, nitrogen, or sulfur), either with or without an active hydrogen atom connected to the heteroatom. The organic compound, in some embodiments, has also been found to be effective in increasing the catalytic activity of modified polymethylaluminoxane which, in addition to methyl, contain C.sub.2 and higher alkyl groups for enhanced solution stability.