Photophysical and photochemical properties for 4,6-diphenyl-3-cyanopyridin-2(1H) thione (CPPT) and 4-phenyl-6-toly-3-cyanopyridin-2(1H)-thione (CMPT) have been investigated by various spectroscopic methods including transient absorption. With changing the solvent polarity, CPPT and CMPT exhibited the thione-thiol tautomeric equilibria. Laser flash photolysis studies in polar solvents and protic solvents have revealed that triplet state formation was predominantly observed by the thione tautomer, which was supported by the characteristic self-quenching in the rate constants of (2.99-5.68)x10 9 dm 3 mol - 1 . In nonopolar and less polar solvents, on the other hand, the thio radical was observed by photoinduced homolytic S H bond cleavage of the thiol form. Absorption and fluorescence spectra indicate the considerable shifts with the solvent polarity in each form in addition to the equilibria. From the fluorescence lifetimes, the tautomeric forms can also be discriminated.