Cp(2)Zr((CH3)-C-13)(2) (1) has been used as a probe for the reactivity of metallocene-methylaluminoxane catalysts for olefin polymerization. A H-1 and C-13 NMR study of the reaction equilibria between Cp(2)Zr((CH3)-C-13)(2) and Lewis acids such as AlMe(3) (2), B(C6F5)(3) (3), and methylaluminoxane (MAO) (4) has been performed. AlMe(3) is always present in MAO solutions, and B(C6F5)(3) is a relatively strong Lewis acid, which has a capacity to form and stabilize ion pairs comparable to that of MAO. The use of isotopically C-13-enriched dimethylzirconocene has permitted the study of these systems by C-13 NMR in conditions as close as possible to usual polymerization conditions, which require large excesses of MAO for reaching high activities. The comparisons of the reactivity of Cp(2)Zr((CH3)-C-13)(2) with B(C6F5)(3) and with MAO have provided the first direct evidence of the formation in solution of monomeric [Cp(2)Zr((CH3)-C-13)](+)[Me . MAO](-) (8), of dimeric [Cp(2)Zr((CH3)-C-13)](2)(mu-(CH3)-C-13)(+)[Me . MAO](-) (7), and of the [(Cp(2)Zr(mu-Me)(2)AlMe(2)](+)[Me . MAO](-) (9) cationic species, having MeMAO(-) counterions. The influence of temperature, Al/Zr mole ratio, and zirconium concentration on the equilibria of ion pair formation has been elucidated.