This part of ISO 2871 specifies a method for the determination of low-molecular-mass cationic-active materials such as monoamines, amine oxides, quaternary ammonium compounds and alkylpyridinium salts which have a main chain of 10 to 22 carbon atoms and not more than 6 other carbon atoms in the cation. The method is also suitable for other cationic-active materials. The method is applicable to solids or to aqueous solutions of the active material when the relative molecular mass of the cationic-active matter is known or when it has been previously determined if its content is expressed as a percentage by mass. If more than one type of cationic-active material is present, an estimate of average relative molecular mass may be used. The method is not applicable if anionic and/or amphoteric surface active agents are present.