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The reduction of a constrained‐geometry catalyst (CGCTiMe2) activated with a series of cocatalysts, including modified methylaluminoxane (MMAO), tris(pentafluorophenyl) borane [B(C6F5)3], and combined B(C6F5)3/MMAO, was experimentally investigated using an on‐line electron spin resonance spectroscopy (ESR) technique. The effects of the solvent type, cocatalyst/catalyst ratio, and temperature on the reduction were examined. In the CGCTiMe2/MMAO system, increasing the Al/Ti molar ratio from 20 to 250 significantly increased the Ti(III) content. Adding trimethylaluminum to the system lowered the Ti(III) content. Three trivalent Ti species, Ti(a) (g = 1.972), Ti(b) [g = 1.992, α(H) = 7.4 G], and Ti(c) (g = 1.995), were observed. In the CGCMe2/B(C6F5)3 system, only one trivalent Ti species (g = 1.988) was observed. The CGCTiMe2/B(C6F5)3 system was more stable to reduction than the CGCTiMe2/MMAO system.