Shortly after the termination of the last war information became available on certain organic phosphorus compounds which had been developed in Germany as insecticides. The products which received first recognition and use in this country are now known as hexaethyl tetraphosphate (HETP), tetraethyl pyrophosphate (TEPP) and parathion. The first two of these are mixtures of esters, the principal active insecticidal agent of which is tetraethyl pyrophosphate. The third, parathion, is an ester of thiophosphoric acid. HEXAETHYL TETRAPHOSPHATE AND TETRAETHYL PYROPHOSPHATE Hexaethyl tetraphosphate is a mobile liquid obtained by the reaction of three moles of triethyl phosphate and one mole of phosphorus oxychloride. From its empiric composition (C2H5)6P4O13, it was concluded that the product was an ester of tetraphosphoric acid. It was subsequently shown in this country that the reaction product is essentially a mixture of esters containing ethyl metaphosphate, triethyl orthophosphate and