A new method is described for the determination of particle size distribution of slurries based on ultrasonic velocity spectrometry combined with gamma-ray transmission. This method shares the advantages of ultrasonic attenuation spectroscopy of being capable of analyzing highly concentrated samples without dilution. However the ultrasonic velocity method is better suited to fine particles of diameter from about 0.1 to 30 mu m, a greater volume of slurry is analysed and therefore sampling errors are reduced, and precise theoretical models are readily available to permit the accurate determination of size distribution by inversion of ultrasonic velocity measurements. The method can also be used to accurately determine particle size cut points by linear correlation. Using either inversion or correlation methods, the accuracy of particle size information from ultrasonic velocity spectroscopy is significantly enhanced by the independent measurement of solids loading by gamma-ray transmission. In addition, larger sizes can be measured by combining the ultrasonic velocity method with ultrasonic attenuation measurements. The method has been tested in the laboratory on a wide variety of mineral and paint slurries, The method determined the size distribution of single component silica and alumina samples in water in agreement with laser diffraction measurements and the method successfully distinguished well and poorly dispersed TiO2 suspensions. For composite samples the method discriminated separate TiO2 and CaCO3 components and determined their proportions to within 0.25% volume, In addition the method, in combination with ultrasonic attenuation measurements, determined the size fractions of iron ore slurries below 10 and 30 mu m to within 1.3% and 1.0% relative respectively, when compared with laser diffraction measurement of particle size, The CSIRO is presently designing on industrial gauge which will be manufactured and installed in an industrial slurry stream in 1997.