Magnetic field effects on the photochemical electron-transfer reactions of 10-methylphenothiazine with 1,4-dicyanobenzene and tetrafluoro-1,4-dicyanobenzene are investigated by a nanosecond laser photolysis technique. For these reactions, the recombination of the geminate radical ion pairs as well as the yields and decay rates of the escaped radical ions showed clear magnetic field dependence in nonviscous homogeneous solutions. The mechanisms of the magnetic field effects are ascribed to the hyperfine coupling and Delta g mechanisms at low and high magnetic fields, respectively. The smaller magnetic field effects on the reaction of the fluorinated derivative than those of 1,4-dicyanobenzene are ascribed to its smaller back-electron-transfer rate because its free-energy change is smaller than that of the nonsubstituted one.