The reduction of a constrained-geometry catalyst (CGCTiMe(2)) 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 CGC-TiMe2/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, alpha(H) = 7.4 G], and Ti(c) (g = 1.995), were observed. In the CGCMe(2)/B(C6F5)(3) system, only one trivalent Ti species (g = 1.988) was observed. The CGCTiMe(2)/ B(C6F5)(3) system was more stable to reduction than the CGCTiMe(2)/MMAO system. (C) 2003 Wiley Periodicals, Inc.