An indirect method for the determination of OsO4, in the p.p.m. region is possible by measuring the decrease in fluorescence of the 4,6-bismethylthio-5-aminopyrimidine when OsO4, is added. At liquid N2temperatures 10 p.p.b. OsO4, can be detected. The OsO4, to ligand ratio was found to be 2:3. For 1 p.p.m. OsO4, 10 p.p.m. Ir+3, 1 p.p.m. Fe+3and less than 1 p.p.m. Ru+3and Pd+2can be tolerated. The fluorescent system is not affected by pH in the 3–12 ranges. It is suggested that the decrease in fluorescence results from a complexation reaction with the first site of reaction being the NH2. group. With increasing OsO4, concentration, oxidation appears to occur which produces a black product. This oxidation was not observed at p.p.b. concentrations of Os.