Equilibria between gaseous oxygen (from 870 to 20 000 Pa) and several high surface area Ce1−xZrxO2 powders (0≤x≤0.36) were studied by means of thermogravimetry in order to get a better insight into the nature and behaviour of the point defects in these solid solutions. By measuring the weight changes between two different oxygen pressures, we could obtain the variations of the oxygen vacancy concentration at the surface of the solutions as a function of the oxygen pressure and zirconium concentration. The anionic sublattice was shown to follow an ideal thermodynamic behaviour, the oxygen vacancy concentration being proportional to PO2−1/6. By comparing the experimental and theoretical variations of the oxygen vacancy concentration versus the zirconium concentration, we found that the cationic sublattice was well described by a nonideal (regular) model in which the activity coefficients of the structural elements CeCex, ZrCex and of the associated defect (ZrCe, CeCe′)′ are different from 1.