An analytical method is described whereby measurement of the rate of sensitized photooxygenation of tetramethylethylene in solution permits precise determination of the various parameters that combine to control the quantum yield for formation of singlet molecular oxygen in solution. Experiments are made in the presence of varying concentrations of dissolved oxygen and a halogenated hydrocarbon that quenches fluorescence emitted by the sensitizer. The method is applied first to the case of methylene blue as sensitizer in order to develop this system for use as a primary standard. The quantum yield for formation of triplet methylene blue in methanol solution, regardless of the concentration of dissolved oxygen but in the absence of a halogenated hydrocarbon, was found to be (0.50 +/- 0.02). The method is subsequently applied to chlorophyll a sensitized reactions, where the triplet quantum yield is determined to be (0.48 +/- 0.03) in deoxygenated benzene solution but increases with increasing concentration of O-2. The procedures presented here permit determination of precise triplet quantum yields together with the parameter that describes the efficacy with which the triplet reacts with O-2 to produce O-2((1) Delta(g)), using steady-state illumination, for a wide variety of sensitizers. It is also shown that, at least for the sensitizers considered here, the methodology permits determination of the fraction of those excited singlet states quenched by O-2 that lead to formation of the corresponding excited triplet state.