Abstract: The invention relates to a method and apparatus for determining the concentration of a substance which is bonded to particles in a flowing medium, where said substance can be present in varying concentrations, a light beam (I.sub.0) being transmitted in the medium and light (I.sub.r) directly reflected from the particles being detected for obtaining a signal which is proportional to the intensity of the reflected light, said signal being used as a measure of the concentration of the substance conditional on particle concentration and opticle properties of the particles being kept constant. Detection is carried out without light passing straight through the medium, by the particle concentration of the sample taken being selected sufficiently great so that the quantity of directly reflected light will be maximum.