At the CaWO4 / WO3 interface in air at 900 degreesC under a minimum external mechanical pressure, two oppositely directed processes-(i) spreading of WO3 on the surface of CaWO4 and (ii) penetration of calcium from CaWO4 (or CaWO4 itself) into the bulk of WO3-occur simultaneously. The electric field of a certain polarity accelerates both processes. To explain the experimental data, a model of the surface intermediate phase (CW-s) was proposed and proved in experiments. The key point is that the specific surface energy alpha (CW-s) is lower than alpha (WO3) and alpha (CaWO4).