A method for the determination of alkyl benzenesulfonates in surface water and sewage is based upon the isolation of the alkyl benzenesulfonate from interfering materials by solvent extractions of its l-methylheptylamine salt, and measurement of the isolated benzenesulfonate bs an improved methylene blue method.The procedure will measure less than 1 p.p.b. in water and 50 p.p.b. or less in sewage, It is not subject to any known interference from materials likely to be present in surface water or sewage.