CSIRO has developed an ultrasonic attenuation and transit time technique and a microwave transmission technique for on-line determination of the mass flow of pneumatically conveyed solids. In a comparative plant trial on a single burner line at Bayswater power station. the ultrasonic technique determined the coal density and velocity with rms errors of 3.9 and 5.9% relative respectively; the microwave technique successfully determined the coal density with an rms error of 6.2% relative, but was unable to accurately determine solids flow velocity at the low coal densities typical of the trial. In a plant trial at a direct smelter, the microwave technique successfully determined the mass Row of pneumatically conveyed iron ore lines feeding the smelter. with an rms error of 10.2% relative.