the photoreduction of methylene blue was carried out in presence of various reducing agents at different acidities. Acidities of the solutions were determined by Hammett acidity function (Ho). The quantum yield of the reaction varies directly with the increase in the concentration of the reductants and temperatures. The results and experimental findings obtained were also interpreted in terms of comparative reaction mechanism associated with reported work, in terms of structure and concentration of reducing agents. From the present investigation it has become more certain that the quantum yield is essentially controlled by two equilibria. i. The triplet state of methylene blue with the proton and the protonated triplet state of methylene blue. ii. The protonated triplet state of methylene blue with reductants, and associated complex of methylene blue. Since the quantum yield is directly proportional to the rate constant and varies with temperature, Arrhenius and Eyring relationships were used to calculate activation parameters such as energy of activation (Ea), free energy change of activation , enthalpy change and entropy change of activation.