Magnetic field effects on both the quenching of the tripler excited state of 10-methylphenothiazine (S = 1) with 4-(4-cyanobenzoyloxy)TEMPO (S = 1/2) and the dynamic behavior of generated radical ion pairs (S = 1/2 and 3/2) were directly observed by a nanosecond laser photolysis technique, in 2-propanol, with increasing field from 0 T to 2 T, the quenching rate was decreased by (11.8 +/- 1.2)% and the yield of the escaped cation radical was increased by (90 +/- 8)%. (C) 1998 Elsevier Science B.V.