A differential spectrophotometric method of assay for hexachlorophene in liquid soaps has been developed. Measurement of the absorbance, at 312 millimicrons, of a dilution of the sample at pH 8 in the solute cell vs. the same dilution atpH 3 in the solvent cell of the spectrophotometer eliminates irrelevant absorption due to the soap base and permits assay of the sample without removal of the soap. The method is proposed for assay of the U. S. P. XV hexachlorophene liquid soap.