Ethylene and propylene were polymerized by different combinations of metallocenes and aluminoxanes. Ethyl-iso-butylaluminoxane (EBAO), which could be easily obtained by direct hydrolysis of Et3Al/iso-Bu3Al in toluene with water, displayed as good polymerization activity as methylaluminoxane when it was paired with some metallocenes. The UV-visible spectroscopic study on different metallocene/aluminoxane pairs suggests that tightness of the ionic pairs generated from the reaction between metallocene and aluminoxane is different for a fixed metallocenium. The loose ionic pair derived from EBAO prefers the monomer propagation and hinders the interaction between active species, which results in high catalytic activity and molecular weight. (C) 2003 Wiley Periodicals, Inc.