Measurement of density and mass flow of disperse systems with a gas as fluid phase flowing through a pipe is carried out by introducing sound in the frequency range 20-2,000 Hz in the middle of the cross-section of the pipe. The sound has a wavelength which is at least half the pipe diameter. The speed propagation of sound waves in the flowing disperse system is is measured in the axial direction of flow over at least two transit sections of differing length; and the speed propagation of sound waves in the flowing disperse system is also measured in a direction contrary to the axial direction of flow over at least two transit sections of differing length. The fill level of a reception member in communication with a line of the disperse system is calculated from the measured mass flow rate over a period of time.