Natural and synthetic anionic surface-active substances may be determined as methylene-blue active substances (MBAS); they are referred to as MBAS-index, a summary parameter method. Under the experimental conditions, sulfonates and sulfates are the compounds chiefly measured. The method is applicable to drinking water, surface water as well as waste water, e.g. for the determination of the primary degradation of surfactants being under investigation in test systems containing natural or synthetic waste water. This applies for both laboratory scaled and technical waste water