Doses of different washing agents are added to the washing liquor during the same washing operation but at different washing steps. Washing or bleaching agents are used which emit fluorescent radiation each in a different wavelength range when exposed to light. Light is brought by an optical waveguide to the measurement sites located in the washing liquor, and the light emitted therein is picked up and led by the same or another optical waveguide to a receiving and evaluating unit. The intensity of the fluorescent radiation is detected in one or several different wavelength ranges and the concentration of the used washing or bleaching agents in the washing liquor is determined therefrom by calibration of said agents. When several different washing agents are used in the same washing liquor, it is possible to determine their individual concentrations.
