H-1 and C-13 NMR studies of the reactions between two beta-alkyl-substituted aluminoxanes obtained from the in situ reaction between Al(i-Bu)(3), Al(i-Oct)(3) and H2O with two metallocenes Cp2ZrCl2 and Cp;ZrCl2 (Cp = cyclopentadienyl, and Cp * = pentamethyicyclopentadienyl) have been performed. The beta-branched aluminoxanes were obtained from the in situ reaction between Al(i-R)(3) and H2O at Al/H2O molar ratio of 2 (R = i-Bu (TIBAO), R = i-Oct (TIOAO)). A comparison of the Cp2ZrCl2 and Cp-2* ZrCl2 reactivity with MAO, as the reference point for aluminoxanes, as well as with their parent aluminum alkyls AlM3, (TMA), Al(i-Bu)(3) (TLBA), and Al(i-Oct)(3) (TIOA) has been made. Cp-2* ZrCl2, because of steric hindrance, is less reactive than Cp2ZrCl2 towards all aluminium compounds considered. The Cp-2*Zr-i-BuCl and Cp-2* Zr-i-OctCl produced appear to be more stable than Cp2Zr-i-BuCl and Cp2Zr-i-OctCl due to the Cp* steric hindrance, which inhibits beta-hydrogen eliminations. The lower reactivity of Cp-2*ZrCl2 along with the higher stability of the zirconium alkyls produced has allowed us to produce evidence of the greater capacity of TIOAO with respect to TIBAO for yielding alkylated ion pairs. These results explain the polymerization activities yielded by metallocenes when combined with the above aluminoxanes, that is, the Cp2ZrCl2 activity only with MAO, and the high polymerization activities of substituted metallocenes when beta-alkyl-substituted aluminoxanes are used as cocatalysts.