The {xCo(2)SiO(4) + (1 - x)Ni2SiO4} solid solutions were studied at the temperatures of (1073, 1173 and 1273) K by solid-state galvanic cells involving beta"-alumina electrolytes conducting cobalt and nickel divalent ions, respectively. The activities of both components of the solid solution were determined directly from the e.m.f. measurements. The solutions exhibited positive deviations from Raoult's law and could be characterized as regular solutions.