Binary mixtures Of CP2ZrMe2, Cp2ZrCl2, dimethylaluminum chloride, trimethylaluminum, and methylaluminoxane (MAO), as well as CP2ZrMe2 with boron-based activators, have been studied by in situ IR spectroscopy (Cp = cyclopentadienyl, Me = methyl). The position of a strong band near 800 cm(-1), corresponding to the out-of-plane vibration of the Cp hydrogen atoms, is sensitive to the bonding environment around Zr and can be used to monitor reactions and the formation of new products in these mixtures. Harmonic frequencies determined by density functional theory correlate well with experimental values and have been used to assist in the interpretation of the data. The frequency of the Cp out-of-plane vibration, ranging from 797 to 832 cm(-1) in our experiments, is found to increase with increasing electron density on the Cp ring and decreasing Zr-Cp distance. In the mixture of MAO and (CPZrMe2)-Zr-2, a stable complex is rapidly formed at low AI/Zr ratios. A mechanism that may explain the need for a large MAO excess is proposed for the activation of metallocenes with MAO. The proposed mechanism involves the formation of dimers or oligomers of MAO cages that tend to dissipate the charge of the anion, This destabilization of the CP2ZrMe2-MAO complex facilitates the formation of the catalytically active cation.