The olefin polymerization catalyst based on Cp(2)Ti(13)CH(3)Cl/MAO has been studied. Reaction equilibria and polymerization of C-13-enriched ethylene were followed by C-13 NMR spectroscopy, and the catalytic activity was determined. Evidence has been obtained that MAO alkylates the transition metal and produces stable cationlike species Cp(2)TiMe(+)Cl .[Al(Me)O](-)(n) (3). Complex 3 has been shown to be active for olefin polymerization. Two types of ion pairs have been detected for the low Al/Mt ratios. We assume that they are more or less tight ion pairs. It has also been possible to relate polymerization activity to the composition of the catalyst mixture. Temperature (-78 divided by -20 degrees C), Al/Mt mole ratios (10 divided by 40), and catalyst dilution have been shown to influence the concentration of active species such as 3 and thus to affect polymerization activity. Dilution decreases the formation of nonactive species such as Cp(2)Ti(Me)-OAl(Me)-MAO (4).