2-Mercaptothiazoline (2MT) is a more toxic as compare to the thiazole and the photosensitized reaction of 2MT with methylene blue (MB) in the alkaline medium using visible light has been studied to convert 2MT to less toxic thiazole. 2MT shows the λmax at 270 nm between the pH range 2 – 6 and the λmax at 235 nm in the pH range 8 – 12. The irradiation of MB molecule with visible light excites it to singlet excited state which gives triplet excited state on ISC which interacts with 2MT anion and transfer it’s energy to 2MT anion and comes back to ground state (dipoledipole energy transfer mechanism). The triplet excited state of 2MT anion gives thiazole radical and sulphur radical anion. The thiazole radical will abstract hydrogen from a hydrogen donor to form thiazole. The sulphur radical anion will react with a proton to form a SH° radical. As the reaction precedes the thiol form is converted to anion form. The apparent rate of the reaction has been calculated and the effect of pH, concentration of the sensitizer, the light intensity on the apparent rate of the reaction has been studied. The quantum efficiency of the photochemical reaction is calculated using potassium ferri oxalate actinometer and the effect of the concentration of the substrate on the quantum efficiency is calculated. The reaction mechanism and the excited states involved have been suggested.