The activities of CaO and LaCrO3 in {xCaO + (1 - x)LaCrO3}, 0 < x less than or equal to 0.4, have been determined in the temperature range 1073 K to 1273 K by solid-electrolyte galvanic cells. Composite ionic conductors based on CaF2 and Ca-beta "-Al2O3 were applied as solid electrolytes. It was found that the solid solutions formed within the composition range 0 < x less than or equal to 0.25 exhibited strong positive deviation from ideal behaviour.