The variation in the oxidation state of the titanium species after reaction with various alkylaluminiums was investigated by X-ray photoelectron spectroscopy (XPS). The reaction of an MgCl2-supported Ziegler catalyst with alkylaluminium was observed to induce a chemical shift to lower binding energy and a broadening of the full width at half maximum intensity (FWHM), depending upon the kind of alkylaluminium. The polymerization kinetics was examined for slurry and stopped-flow polymerization to estimate the relationship between polymerization stages. The catalyst activation and the decay during the polymerization were found to be related to the variation in the titanium species arising from various alkylaluminium compounds.