Thermally induced transformations in materials (e.g. melting, decomposition or solid-solid phase changes) alter their dielectric properties and hence their ability to interact with a microwave field. This paper describes a new technique, microwave thermal analysis, where microwaves are used both to heat a material and as a means of detecting thermal transitions. Two approaches are described. The first is based on the changes in the temperature of a material when subjected to a constant microwave power and the second on the microwave power profile obtained when a material is heated in a controlled (linear) manner. Both approaches can provide qualitative and quantitative information on solid state processes. A classification is proposed for the different types of results found for various materials and transitions. The advantages and limitations of studying transitions and reactions using microwave energy are discussed. (C) 2000 Elsevier Science B.V.