In order to study the formation and the nature of Ti/Al adducts that play a role in the olefin polymerization activity of the group 4 metallocene-methylaluminoxane catalytic systems, the reactions of AlMe3 and/or methylaluminoxane with Cp2TiMeCl and Cp2TiMe2 have been monitored by H-1 and C-13 NMR spectroscopy. Temperatures between 203 and 283 K and Al/Ti ratios from 1 to 40 have been used. The change in C-13 NMR signals has been found the most diagnostic. Several adducts have been observed and identified. The kind of adducts changes both with the reaction temperature and the Al/Ti ratio. By comparing the MAO and AlMe3 reactions with Cp2TiMeCl it is possible to deduce that, with respect to AlMe3, MAO is a better alkylating agent and it has a greater capacity for producing and stabilizing cation-like complexes. At the Al/Ti ratio of 40 a complex that we tentatively assign as a solvent separated ion pair prevails. The fact that, after the addition of CH2Cl2 to the toluene solution, this complex is present in greater amounts is consistent with this assignment.