The magnetic field effects (MFEs), caused by the Delta g mechanism, on the pehotoinduced hydrogen abstraction reaction of benzopheneone with thiophenol were investigated in alcoholic solutions of varying viscosities (eta = 0.55 to 59.2 cP) by a nanosecond laser flash photolysis technique. The escape yield of benzophenone ketyl radicals (Y) gradually decreased with increasing magnetic field strength (B) from 0 to 1.6 T. The relative yield observed at 1.6 T, R(1.6 T) = Y(1.6 T)/Y(0 T), decreased with increasing eta in the range of 0.55 cP <= eta <= 5 cP, and then increased with increasing eta in the range of 5 cP < eta <= 55.3 cP. When eta was higher than 55.3 cP, the R(1.6 T) value became 1.0, and MFEs were completely quenched. The observed eta dependence of the MFEs was analyzed by the stochastic Liouville equation (SLE), in which the effects of spin-orbit coupling by a heavy atom such as sulfur were taken into account. The observed MFEs were reproduced fairly well by the SLE analysis. The diffusion coefficients of the radicals obtained by the SLE were about three times smaller than those expected from the macroscopic solvent viscosities. One can probe the microviscosity in the vicinity of the radical pairs by observing MFEs on the present photochemical reaction system.