The use of microwaves in the food industry is attributed to the lower time needed to increase the temperature of foodstuffs compared to the traditional heating methods. However, the heating is not uniform and the products show hot and cold spots. In order to analyze the behavior of microwaved foods a mathematical method was developed solving the unsteady state heat transfer differential equations. The model was applied to large systems for which Lambert's law is valid because it leads to similar results as Maxwell equation. It takes into account variable thermal and electromagnetic properties. The numerical solution was developed using an implicit finite difference method in one dimensional systems (sphere, infinite cylinder and slab) and an alterning direction method in two- and three-dimensional conditions (finite cylinders and brick shaped products). It allows to predict temperature profiles and heating times. The model was validated with own data of mashed potato and meat products and with experimental data from literature obtained with agar gel, sodium alginate gel and whole potato. (c) 2004 Elsevier Ltd. All rights reserved.