Hydrogen abstraction reactions of thiobenzophenone with thiophenol in solutions of varying viscosities (eta = 0.29-42.0 cP) were studied by a nanosecond laser flash photolysis under magnetic fields of 0-15.5 T. In alcoholic solutions, the escaped radical yield (Y) of thiobenzophenone ketyl radical showed appreciable magnetic field effects (MFEs). The observed MFEs can be interpreted with the Delta g mechanism through the triplet radical pair. The relative escaped radical yield (R(1.7T) = Y(1.7T)/Y(0T)) decreased with increasing eta at 0 < eta <= 3.33 cP, but then the yield increased with increasing eta at 3.33 cP