On irradiation of 4-aryl-4-methyl-2,6-diphenyl-4H-thiopyrans 4a-4f in methanol solutions with a low-pressure mercury lamp under mild conditions for short time, only aryl groups rearrange selectively to form 2H-thiopyran isomers 6a-6f through their 6-aryl-5-methyl-1,3-diphenyl-2-thiabicyclo[3.1.0]hex-3-ene intermediates 5a-5f in quantitative yields. The intermediates 5a-5f were separated by HPLC from the mixtures of unreacted 4a-4f and photoproducts 6a-6f. Both steps of unimolecular photoisomerization of 4a-4f are investigated from a kinetic point of view using HPLC. The relative rate constants reveal that the formation of intermediate is the rate-determining step in photoisomerization of 4H-thiopyrans 4a-4f to the corresponding 2H-thiopyrans 6a-6f in methanol-d 4 solution. Moreover, comparison of the relative rate constants for the faster step show that the electron withdrawing groups on the para position of the migratory 4-aryl group, unlike those of the electron donating, decrease the conversion relative rates of intermediates to their 2H-thiopyran isomers.