The effect of WO3 activity (a(WO3)) in the surrounding gas phase on spontaneous processes at the CaWO4\WO3 interface was examined in experiments performed in a chamber open to the ambient air (i) and in a closed chamber (ii), where the samples were brought to an equilibrium with the WO3 vapor. The processes in both types of the chambers exhibited a bilateral behavior: spreading of WO3 into the CaWO4 ceramic and, simultaneously, penetration of CaWO4 components into the WO3 ceramic. The mass of the CaWO4 pellets was found to change to a significantly different degree. The conductivity of the CaWO4 ceramic increased with growing a WO3. The obtained results were discussed using a model of formation of the CW-s intermediate surface phase. When a(WO3) increased, the CW-s film became thicker and the degree of the film continuity was enhanced.