Abstract: Simultaneous measuring of the concentration of solids and particle size distribution in a suspension is effected by exciting the suspension by ultrasonic waves of a plurality of frequencies, the wavelength at the lowest frequency being greater than the diameter of the largest particles to be expected and the wavelength of the highest frequency being smaller than the diameter of the smallest particles to be expected. The dimensional spectrum of the solid particles is divided into a plurality of dimensional intervals for which the respective solids concentrations are determined by measuring the radiation absorption of each frequency used for irradiation and representing the same as the sum of the products of the absorption coefficients which are specific of the frequency and dimensional interval with the unknown particle concentrations. This results in a linear system of equations which is solved with respect to the unknown concentrations.