The thermodynamic activities of CaO in the solid solutions which can exist in the system CaO-CdO were determined in the temperature range 823-1303 K by the high temperature solid electrolyte galvanic cell technique with calcium fluoride as the solid electrolyte. The conventional cell arrangement of sandwiching the electrolyte in direct contact with the electrodes was not found to give stable reproducible EMF values at temperatures above 1123 K. For the higher temperatures a modified technique has been introduced, which permits the establishment of an equilibrium through the gas phase. In the temperature range of study, the system was observed to show near ideal behaviour. The results are in agreement with the near zero enthalpies of mixing reported in the literature and also with the theroretical models available for predicting the mixing enthalpies in such systems.