ΔG(O 2) has been measured using the method of physicochemical equilibria between a gaseous oxidoreducing phase and an oxide sample. When CeO 2 oxide is reduced, the ΔG( O 2)( O Ce ) curve first shows a sigmoid part, i.e., the fluorite-type phase field. Second, a horizontal part appears, which indicates the existence of a biphasic field. Third, one has the monophasic field of the rare earth C-type phase. This field is separated from Ce 2O 3 oxide by a very wide horizontal part. A finer study shows that (a) the lower limit of the fluorite-type phase moves toward the reduced compositions when the temperature rises, (b) there exists at 1296 and 1244°K a nonstoichiometric oxide with a formula Ce 7O 12± x , (c) Ce 2O 3 oxide is slightly superstoichiometric. A statistical thermodynamics model has been built up to account for the experimental values of ΔH( O 2)( O Ce ), ΔG( O 2( O Ce ) , and electrical conductivity σ ( O Ce ). Oxygen vacancies in their three states of ionization, V ··, V ·, V x , and the divacancies (VV) x are taken into account. A blocking procedure is used. The predictions for ΔG(O 2) and σ are fairly good. The differences between theoretical and experimental values of ΔH(O 2) never exceed 4 kcal mole −1; however, the typical shape of the experimental curve is not well reproduced.