The decomposition of the carbamate insecticides carbaryl and Mobam and the herbicides IPC and CIPC in water at various pH's was investigated. Analyses were performedin situon silica-gel thin-layer chromatograms by reflectance spectroscopy after the developed chromatograms had been sprayed with the reagent CNTNF (9-dicyanomethylene-2,4,7-, trinitrofluorene) to form visible pi-complexes of the pesticides. Carbaryl and Mobam decomposed rapidly at basic pH's but were stable in acid. IPC and CIPC were stable in water at pH's tested.