The hydrogen abstraction reaction of triplet benzophenone with thiophenol at room temperature was investigated in several nonviscous homogeneous solutions (eta = 0.29-3.33 cP) by means of a nanosecond laser flash photolysis technique under magnetic fields of 0-10 T. The escape yield of the benzophenone ketyl radical decreased with increasing magnetic field strength from 0 to 10 T. In 2-methyl-1-propanol (eta = 3.33 cP), the yield at 10 T reduced to 80 +/- 2% of that at 0 T. The observed magnetic field effects can be interpreted by the Delta g mechanism when the geminate recombination process is faster than or comparable to the escape one.