The method is also suitable for other cationic-active materials. Not applicable if anionic and/or amphoteric surface active agents are present. The sample is titrated in a two-phase (aqueous chloroform) system against a standard anionic surface active agent in the presence of an indicator consisting of mixed anionic and cationic dyes. The cationic surface active agent present in the sample initially reacts with the anionic dye to form a salt which dissolves in the chloroform, imparting a blue colour to this layer. During the titration, the anionic surface active agent displaces the anionic dye and, at the end point, forms a salt with the cationic dye, imparting a greyish-pink colour to the chloroform layer.