The hydrogen abstraction reaction of triplet 4-methoxy-benzophenone with thiophenol at 265 K was studied by a sub-nanosecond laser flash photolysis technique in the absence and presence of a magnetic field of 1.75 T. The decay rate of the generated radical pair at zero field was found to be 1.1 X 10(9) s(-1) and that at 1.75 T to be 1.8 X 10(9) s(-1). The observed magnetic field effect can be explained by the acceleration of the triplet-singlet spin conversion process due to the Delta g mechanism.