Abstract: In order to determine the size category or categories of particles dispersed in a liquid a sample flow is passed through a flow channel of sufficient length to allow, at a certain point, a stable flow with a predetermined flow velocity distribution over the channel cross section, such velocity distribution also causing a distribution of the particles over the channel cross section according to the particle size, whereupon a measure of the size category is obtained by determining the amounts of particles appearing in different areas of the channel cross section. The method for determining the particle amount in a certain area of the cross section comprises use of an X-ray spectrometer, whereby the size category may be determined for different materials selectively, and at a second point of the channel there may be caused a strong turbulence to assure a uniform distribution of the particles, whereby at this second point there is obtained a corresponding normalizing measure for the equalization of the effects e.