The ethylene polymerization activity of rac-Et(indenyl)(2)ZrCl2/AlR3/modifier was studied. Modifiers were added to beta-branched trialkylaluminum Al(i-Bu)(3) (TIBA), and Al(i-Oct)(3) (TIOA), and to their aluminoxanes, TIBAO and TIOAO, respectively, with the aim of obtaining new cocatalysts that operate at low aluminum/transition metal ratios. The modifiers selected contain hetero-atoms and aryl or alkyl groups bearing electron withdrawing substituents or sterically encumbered groups: C6F5OH, C6Cl5OH, C6F4(OH)(2) C6F5NH2, 2,5-(C6H5)(2)C6H3OH, (CF3)(2)CHOH. The in situ reaction between aluminum compounds and modifiers such as C6F5OH yields new modified aluminum alkyl and aluminoxane cocatalysts bearing one alkoxy or aryloxy group. The simultaneous effect of electron withdrawing substituents on the aryloxy group and of steric hindrance due to beta-alkyl substituted alkyls on aluminum leads to good activation of metallocenes at Al/Zr molar ratios as low as 200/1, in conditions in which methylaluminoxane and beta-branched alkylaluminoxanes are inactive. NMR studies of the reactions between C5Me5ZrCl2 with TIOA, TIOA/C6F5OH, TIOAO and TIOAO/C6F5OH demonstrated that in contrast to TIOAO-C6F5OH, TIOA-C6F5OH allows the formation of an active alkylated ion pair, thus qualitatively allowing us to understand the polymerization activity Of C5Me5ZrCl2 based systems. (C) 2003 Elsevier Science B.V. All rights reserved.