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Beroza and Bowman have developed the idea of p-value for confirmation of insecticide identity and for cleanup of insecticides at the nanogram level. In this paper the concept of using the p-value for determination of the parameters of solvent choice for quantitative liquid-liquid extraction of organo-phosphate pesticides from water are reported. The p-value method is modified to suit the present application. The p-value can be used to determine the quantitative extraction of the parent, oxon, and hydrolysis products of Diazinon, Parathion, Malathion, and Baytex. The first step of aqueous residue analysis consists of pesticidal extraction. It is recommended that any pesticide analysis of water should consider the quantitative aspects of the extraction as well as the subsequent determinative steps. Thus, a more complete recovery picture of the pesticide and its environmental fate can be followed.A p-value approach for the determination of the parameters for quantitative liquid-liquid extraction (LLE) of some organophosphate pesticides will be outlined. p-Values have previously been utilized by Beroza (1965, 1966) for confirmation of insecticide identity and for cleanup at the nanogram level. Suffet and Faust (1971) have outlined the theoretical p-value approach for application to LLE of pesticides from aqueous solution. The p-value is defined as "the fraction of the total solute that distributes itself in the nonpolar phase of an equivolume solvent pair" (Beroza et ah, 1969).This paper deals with the selection of pH value and solvent from p-values for organophosphate systems, consisting of the parent molecule, oxon, and hydrolysis products of Diazinon, Baytex (Fenthion), Parathion, and Malathion (Table I).The authors have reviewed the subject of liquid-liquid extraction of organophosphate pesticides from water (Faust and Suffet, 1966, 1972. Serial and continuous extractions of organophosphate pesticides were employed in several papers that were concerned with laboratory and/or field studies. Tables in these reviews showed the available percent recovery data by fortification procedures for several pesticides.The methods of extraction were concerned with one pesticide, a related group of pesticides, or a pesticide and its degradation products. Each procedure arbitrarily utilized a particular solvent and specified the number of extractions and various solvent to water ratios. In some cases, percent recovery, as determined by a technique of fortification, was the only experimental justification for selection of the solvent.Many questions can be asked of these procedures: What dictated the selection of the solvent ? What are the effects of such variables as pH, ionic strength, temperature, and turbid-