Summary. The use of equilibrium thermodynamics in describing interfacial reactions between non-ionic inorganic solids is demonstrated using examples of high-temperature interactions in the Ti–Si–N and Mo–Si–N systems. In the case of a diffusion-controlled process, solid-state reactions can be interpreted with chemical potential (activity) diagrams. The role of volatile reaction products formed during interaction in developing the diffusion zone morphology is analysed. The interfacial phenomena in systems based on dense Si3N4 and non-nitride forming metals can be explained by assuming a nitrogen pressure build-up at the contact surface. This pressure determines the chemical potential of Si at the interface and, hence, the reaction products in the diffusion zone. Keywords. Ceramics; Phase diagrams; Thermodynamics; Transition metals compounds.