In a study of the reaction system MAO/(C5H5)(2)ZrMe2, the size of the ion pair [(C5H5)(2)Zr(muMe)(2)AlMe2](+) [Me-MAO](-) was determined by pulsed field-gradient NMR of its cationic moiety. A mean effective hydrodynamic radius of 12.2-12.5 Angstrom, determined from diffusion rates in benzene solution at different zirconocene and MAO concentrations, indicates that the ion pair remains associated even at the lowest concentrations studied. At elevated concentrations, aggregation to ion quadruples or higher aggregates is indicated by an apparent size increase and by shifts of the C5H5 and Me H-1 NMR signals. The equilibrium constant for the reaction [(C5H5)(2)ZrMe+...Me-MAO(-)] + 1/2Al(2)Me(6) [(C5H5)(2)Zr(mu-Me)(2)AlMe2](+) [Me-MAO](-) changes at different Al/Zr ratios; this indicates that MAO contains various species that produce Me-MAO-anions with different Lewis basicities. The volume of the Me-MAO(-) anion suggests that it contains 150-200 Al atoms.