Before the development of a theoretical background and extensive support of the experimental data photochemistry was thought to be a flimsy subject, devoid of system with a number of processes involved and ill equipped with apparatus but the dawn of quantum mechanics and kinetics elucidate a vast field of research and since then photochemistry has played a vital role in chemistry. Thiazine dyes provide a vast field for the studies of photochemical kinetics in the presence of reductant. Methylene green is a member of thiazine dye which undergoes reduction by the visible light with reductant in presence of buffer solution. Acidity plays a significant role in the reduction of dye. The quantum yield for photochemical reduction used to study the kinetics of reaction which helps to propose reaction mechanism. The triplet excited state of methylene green played a major contribution for the determination of quantum yield. An equilibrium was established between associated complex and one form of triplet state of methylene green with reductant and the complex is the rate controlling specie.