The knowledge of the ice content of frozen systems is of the utmost interest for monitoring and controlling frozen food production and storage. It is also useful if freezing is an undesirable process. Although there is a well known relation between temperature and ice content, this is of little practical help since the easily measurable superficial temperature is not suitable for this calculation: information on the spatial temperature distribution is required. The speed of sound depends strongly on the physical state of media, briskly varying from water to ice. It is also a function of temperature. An easy and fully feasible method to determine a representative overall temperature of food and the ice content by measuring the speed of sound through a given sample thickness is here reported. The method is quick and suitable for online monitoring of frozen, freezing and thawing systems, and it can be adapted to a large variety of containers, geometrical situations and water contents. Data from model and real food systems are presented. (C) 2008 Elsevier Ltd. All rights reserved.