A thermogravimetric study of the oxygen activity dependent variation of the oxygen content in CaO-doped zirconia has been performed at different dopant concentrations within the stability limit of the tetragonal solid solution at temperatures between 1200 and 1300 degrees C. The experimental results are compatible with a simple defect model incorporating oxygen vacancies and dopant cations, not or partially associated with each other. In tetragonal, CaO-doped zirconia, the largest mass change with increasing oxygen activity occurs at high oxygen activities and is due to a component activity dependent disorder.