Ultrasonic sensors have been used to measure particle concentrations and variations in their size distribution in dense liquid-solid suspensions. The measurements are based on variations in velocity, attenuation and mean frequency of the acoustic pulse. Solid loading of up to 45 vol.% and particle sizes of 35, 70 and 180 mum have been used. The observed relationships between the measured parameters and solids concentration are presented. Available theoretical approaches have been reviewed and their applications and limitations have been pointed out for the data of this study. The variation of acoustic velocity and attenuation as a function of slurry concentration are well predicted by the theory for 35 l-Lm particles but not for larger particles. Approaches for determining particle size variations in suspensions are presented.