Abstract: A process is described which comprises heating a methylalumoxane solution in an aromatic hydrocarbon solvent at a temperature of at least 35.degree. C. for at least 0.5 hour in an inert, dry atmosphere such that the resulting heat-treated composition provides, in a supported metallocene catalyst produced using the heat-treated composition, increased activity as compared to the same supported metallocene catalyst produced in the same way except for using a portion of the methylalumoxane solution that has not been heat-treated. Before the heating the methylalumoxane of the solution contains from about 5 to about 35 mole percent unreacted trimethylaluminum, and was formed by partial hydrolysis of trimethylaluminum with free water in an organic solvent.