An on-line electron spin resonance (ESR) technique was applied to investigate the oxidation states of the metallocene catalysts CpTiCl3, CpZrCl3, Cp2TiCl2, and Cp2ZrCl2. These metallocene catalysts were activated by modified methylaluminoxane (MMAO). It was found that the titanocene catalysts (CpTiCl3 and Cp2TiCl2) were readily reduced to the trivalent state while the zirconocene catalysts (CpZrCl3 and Cp2ZrCl2) were quite stable with respect to reduction. The concentrations of the trivalent species Ti(III) and Zr(III) showed linear relationships with the concentrations of metallocene catalyst precursors. However, their slopes were always smaller than unity indicating the existence of bimetallic interactions of the active sites. The ESR detectable Ti(III) and Zr(III) concentrations initially increased with the MAO/catalyst ratio and then leveled off after an 800-1000 Al/catalyst molar ratio. The deactivation processes were followed as a function of aging time over a range of temperature (25-100 degrees C). The decay curves strongly depended on aging temperature with higher temperature giving faster decay rates.