Abstract— The chemiluminescence reaction of luminol has been investigated using conditions of methylene blue photosensitized oxidation. The quantum yields of chemiluminescence obtained were dependent upon temperature and the concentrations of luminol and base; and under the optimum conditions of high temperature and low luminol concentration, the value of the quantum yield approached that for the chemical reaction where the oxidant is hydrogen peroxide and catalyst. An analysis of the results suggests that it is not the primary species produced on photosensitization which is responsible for the chemiluminescent reaction, but a species produced by reaction of the primary species with base.