A method is outlined for the detection of anticholinesterase agents similar to sarin (isopropyl methylphosphonofluoridate) in aqueous solution at concentrations as low as 0.1 p.p.m. in 10 ml. of solution by the use of the Schoenemann reaction. A new series of indicators, 4,4′-(2-thenylidene)bis[N,N-dimethylaniline] variously substituted in the thiophene ring, was used in the investigation. The dependence of color formation on pH, chloride ion concentration, and other factors led to an explanation of the mechanism of the reaction involved. Chloride ion is necessary for the reaction and, in the absence of the indicator, free chlorine can be detected in acid solution. Presumably, then, during acidification perphosphonate ion reacts with chloride ion to give hypochlorous acid, which oxidizes the mono ion of the leuco compound to a dye.Ions such as cyanide ion, with a lower oxidation–reduction potential than the leuco compounds, interfere with the test. Oxidation of the leuco compound by hypochlorous acid is bypassed in the method described for the detection of sarin in the presence of hypochlorous acid.