Solid state spreading is experimentally investigated in three types of systems: "ionic salt MX-oxide", "ionic oxide-ionic oxide", and "covalent oxide-ionic oxide", each differing from one another in their nature of chemical bonding (bond ionicity). The novel data obtained in the study clearly show that the mechanisms of solid state spreading are essentially different in these types of systems and depend upon the surface energy and on the type of chemical bonding of the contacting phases. The main factors governing this process are chemical bond ionicity in the mobile, spreading phase and chemical affinity between the mobile phase and the substrate. The mechanism of solid state spreading of an ionic salt over the supporting oxide proceeds via the formation of a non-autonomous interfacial phase (interphase) MX-s. For interfaces of type "covalent, soft oxide I ionic oxide", the spreading mechanism is complicated. A non-autonomous interphase is first of all formed at the interface. This phase is characterized by bilateral surface activity and mobility. It then spreads over the grain surface of both phases in contact. In the case of the MeWO4 vertical bar WO3-interface, a MeW-s interphase is formed and exists at elevated temperatures. A set of experimental evidence of MeW-s formation and bilateral movement is obtained. MeW-s possesses rather high electrolytic conduction. The latter is the reason of metacomposite solid electrolyte formation and is a trigger for the field-assisted spreading and a fast electro-surface counter-transfer of MeWO4 and WO3. (c) 2006 Elsevier B.V. All rights reserved.