A simple and precise colorimetric method of determining thiosulfate and tetrathionate is described. It involves alkaline cyanolysis of thiosulfate S in the presence of Cu2+or alkaline cyanolysis of tetrathionate S in the absence of Cu2+and colorimetric determination of the thiocyanate formed as ferric thiocyanate complex. The method is rapid, sensitive, and accurate, and it permits determination of microgram quantities of thiosulfate or tetrathionate or both. The stability of the color formed is discussed, and the ratio of tetrathionate recovered in the presence and absence of Cu2+is presented.