Magnetic field effects (MFEs) on reactions of biradical radical ion pairs (BRIPs, S = 3/2 and 1/2) were investigated for the electron transfer (ET) reactions of the triplet State of 10-methylphenothiazine ((3)MPTZ*) (S = 1) and electron accepters linked with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) (An-R-.) (S 1/2) in various solvents by means of a nanosecond laser photolysis technique. In 2-propanol, the yield of free ions dramatically increased with increasing magnetic field (B) from 0 to 2 T, but slightly decreased from 2 to 10 T. The magnitude of the MFEs was much larger than those observed in ET reactions of 3MPTZ* with acceptors without TEMPO (S = 0). To interpret the MFEs observed in the reactions with An-R-,(.) we considered the following two factors. (i) In the quenching of (3)MPTZ* by An-R-. not only the ET reaction but also triplet-doublet (T-D) quenching takes place. The T-D quenching efficiency decreases with increasing B, resulting in a higher yield of BRIPs ([MPTZ(.+) An(.-)-R-.]). However, this factor was found to have only a small contribution to the observed MFEs on the free ion yield. (ii) The ET reactions generate (4.2)[MPTZ(.+) An(.-)-R-.] in quartet (Q) and doublet (D) states, which decay through either the spin-selective back ET from the D states or the separation to free ions. At B = 0 T, the spin conversion between the Q and D states is efficient. The increase in the free ion yield with increasing B from 0 to 2 T can be attributed to the spin relaxation from the Q(+/-3/2) states to the Q(+/-1/2) and D+/-1/2 states due to the dipole-dipole interaction of (3)(An(.-)-R-.). The slight decrease in the free ion yield with increasing B from 2 to 10 T can be attributed to acceleration of the relaxation induced by the anisotropic Zeeman interaction and/or enhancement of the Q(+/-1/2)-D+/-1/2 conversion induced by the difference in the isotropic g-factor between MPTZ(.+) and An(.-)-R-.. Effects of solvents and additives on the present MFEs were also studied.