A homemade instrument for the measurement of oxygen concentration in aqueous solutions measures the decay rate of the phosphorescence of a Pd-porphyrin complex (phosphor) dissolved in the solution, which is flashed every 0.1 s with 630 nm light. The concentration Of 02 is a linear function of the decay rate. The instrument is used to study the reaction of dithionite (S2O42-) with O-2 at 25 degrees C and 37 degrees C. It is found that the ratio of dithionite to oxygen consumed in the reaction is 1.2+/-0.2 at 25 degrees C and 1.7+/-0.1 at 37 degrees C, suggesting a temperature-dependent stoichiometry. At both temperatures, the initial rate Of 02 consumption, -d[O-2]/dt, is found to be 1/2 order in S2O42- and first order in O-2. This finding is consistent with a previously proposed mechanism: S2O42- <-> 2SO(2)(-) comes to a rapid equilibrium, and SO2- reacts with O-2 in the rate-determining step.