Cation-active tensides of the dialkyldimethylammonium chloride type may be separated from a-tensides and n-tensides and determined photometrically after their reaction with bromophenol blue or disulphine blue. In the presence of clay minerals or of materials with similar activity, however, they are so firmly bound that their analytical determination in water and waste water is not possible any more. In practice, disulphine blue substances have the misleading appearance of being present as cation-active tensides.