A dynamic model was developed describing dielectric heating dynamics of a heterogeneous material, in which the dispersed phase heats more quickly than the surrounding phase. The model includes effects of scattering by small particles, absorption and differences in thermal and conduction properties, as well as domain size. The model shows that the smallest domain sizes that can be heated selectively are determined by dielectric and thermal properties of both domains and the applied field strength. In the case of food products, it can be concluded that the minimal size at which significant temperature differences can occur is about 1 mm applying current available microwave technology. However, in the case of pulsed fields, the domains that can be heated selectively can be orders of magnitude smaller than with constant fields, without requiring higher overall energy inputs. (C) 2003 Elsevier Ltd. All rights reserved.