The time of flight of an ultrasonic pulse moving parallel to the direction of heat flux was measured in blocks of food (gelatin, chicken, beef) during freezing. Echoes were recorded from the food surfaces and from the moving ice front within the food. The changing return times of these echoes were used to calculate the percentage of the food frozen as a function of time either from the measured position of the ice front or by assuming the overall time of flight of the acoustic pulse was the sum of times spent traveling through a frozen and unfrozen phase. Both methods are predicated on the observation that for foods the change in speed of sound on freezing is much greater than the changes with respect to temperature in the absence of a phase change. Both methods gave reasonable prediction of the time for complete freezing as measured by thermocouples but the latter was more practical as the ice front was difficult to measure as the muscle foods became increasingly frozen. (C) 2003 Elsevier Ltd. All rights reserved.