An ultrasonic transmission system was used to measure transmission of ultrasonic waves through a coagulating milk system. Reconstituted skim milk was coagulated at different temperatures and rennet and calcium chloride concentrations. Ultrasonic wave diffusivity was determined based on experimental velocity and attenuation data. Wave diffusivity increased with decreasing milk fluidity as coagulation progressed. A finite difference model was developed to predict ultrasonic diffusivity based on the wave and diffusion equations. The numerical predictions compared well with the experimentally determined diffusivity values. However, the numerical values were always lower than their corresponding experimental values. This difference was apparently a result of energy loss at the transducer-milk gel interface, which was assumed to be zero for the numerical model. (C) 2002 Elsevier Science Ltd. All rights reserved.