Using 1 Hand 13 C-NMR spectroscopies, cationic intermediates formed by activation of L 2 ZrCl 2 with methylaluminoxane (MAO) in toluene were monitored at Al/Zr ratios from 50 to 1000 (L 2 are various cyclopentadienyl (Cp), indenyl (Ind) and fluorenyl (Flu) ligands). The following catalysts were studied: (Cp-R) 2 ZrCl 2 (R0/Me, 1,2-Me 2 , 1,2,3-Me 3 , 1,2,4-Me 3 , Me 4 , Me 5 , n-Bu, t-Bu), racethanediyl(Ind) 2 ZrCl 2 , rac-Me 2 Si(Ind) 2 ZrCl 2 , rac-Me 2 Si(1-Ind-2-Me) 2 ZrCl 2 , rac-ethanediyl(1-Ind-4,5,6,7-H 4) 2 ZrCl 2 , (Ind-2-Me) 2 ZrCl 2 , Me 2 C(Cp)(Flu)ZrCl 2 , Me 2 C(Cp-3-Me)(Flu)ZrCl 2 and Me 2 Si(Flu) 2 ZrCl 2. Correlations between spectroscopic and ethene polymerization data for catalysts (Cp-R) 2 ZrCl 2 /MAO (R 0/H, Me, 1,2-Me 2 , 1,2,3-Me 3 , 1,2,4-Me 3 , Me 4 , Me 5 , n-Bu, t-Bu) and rac-Me 2 Si(Ind) 2 ZrCl 2 were established. The catalysts (Cp-R) 2 ZrCl 2 /AlMe 3 /CPh 3 ' B(C 6 F 5) 4 ((R0/Me, 1,2-Me 2 , 1,2,3-Me 3 , 1,2,4-Me 3 , Me 4 , n-Bu, t-Bu) were also studied for comparison of spectroscopic and polymerization data with MAO-based systems. Complexes of type (Cp-R) 2 ZrMe ' 1/Me (-Al /MAO (IV) with different [Me-MAO] (counteranions have been identified in the (Cp-R) 2 ZrCl 2 /MAO (R 0/n-Bu, t-Bu) systems at low Al/Zr ratios (50 Á/200). At Al/Zr ratios of 500 Á/1000, the complex [L 2 Zr(m-Me) 2 AlMe 2 ] ' [Me-MAO] ((III) dominates in all MAO-based reaction systems studied. Ethene polymerization activity strongly depends on the Al/Zr ratio (Al/Zr 0/200 Á/1000) for the systems (Cp-R) 2 ZrCl 2 /MAO (R 0/H, Me, n-Bu, t-Bu), while it is virtually constant in the same range of Al/Zr ratios for the catalytic systems (Cp-R) 2 ZrCl 2 /MAO (R0/1,2-Me 2 , 1,2,3-Me 3 , 1,2,4-Me 3 , Me 4) and rac-Me 2 Si(Ind) 2 ZrCl 2 /MAO. The data obtained are interpreted on assumption that complex III is the main precursor of the active centers of polymerization in MAO-based systems.