Magnetic field effects (MFEs) due to the isotropic ag mechanism were studied for the hydrogen abstraction reactions of 4-methoxy-benzophenone with thiophenol in several solvents at 293 K by a laser flash photolysis technique under ultrahigh magnetic fields of up to 30 T. In 2-propanol, ethanol, and methanol, the yield of the escaped ketyl radical decreased with increasing magnetic field from 0 to 30 T. In 2-methyl-1-propanol, however, the decrease was almost saturated at 20 T and the yield above 20 T reduced to be 2/3 Of that at 0 T. Such saturation of the MFEs due to the isotropic Delta g mechanism in the reaction through radical pairsin solution was found for the first time.