Abstract-Hydrolysis of O-ethyl O-p-cyanophenyl, phenyl phosphonothionate (Cyanofenphos) has been studied in the acidic medium with catalyst such as Zr 4+ , Ce 4+ , Hf 4+ and Th 4+ cations, and the rate maxima observed were at around pH 2.0. Since the metal aquo complex are known to exist in equilibrium with deprotonated species, and therefore metal bound OHnucleophile might be available abundantly even in acidic aqueous solutions. Thus an interaction of sulphur of P=S group with metal cation followed by an intramolecular nucleophilic attack of hydroxy anion at the ester phosphorus atom would results in the cleavage of P-O bond.