This part of ISO2871 specifies a method for the determination of high-molecular-mass cationic-active materials such as quaternary ammonium compounds in which two of the alkyl groups each contain 10 or more carbon atoms, e.g. distearyl-dimethyl-ammonium chlorides, or salts of imidazoline or 3-methylimidazoline in which long-chain acylaminoethyl and alkyl groups are substituted in the 1- and 2-positions, respectively. 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. The method is not applicable if anionic surface active agents are present.