An on-line electron spin resonance (ESR) technique was applied to investigate the syndiospecific polymerization of styrene activated by the catalyst system CpTiCl3/MMAO. The measurements included trivalent titanocene concentration and monomer conversion. The activation procedure was found to have a dramatic effect on catalyst activity. Adding the reactants in the order of (MMAO + CpTiCl3) + St gave a much higher trivalent titanocene concentration and catalyst activity than the order of (MMAO + St) + CpTiCl3. The catalyst deactivation behaviors in the temperature range of 25-70 degrees C were followed as a function of time during polymerization. At high Al/Ti ratios (500-1000), the decay rates of trivalent titanocene in the presence of styrene were much faster than those of the pure catalyst system.