In order to investigate the thermodynamic behavior of t＇, t*, and k phases with the composition, CeZrO4, the equilibrium oxygen partial pressure, p(o2), over their mixtures with pyrochlore (Ce2Zr2O7+x) was measured utilizing an electrochemical cell : Pt, {CeZrO4(t＇, t*, or k phase) + Ce2Zr2O7+x}/ZrO2(+Y2O3)/air, Pt. The conclusions described below were derived : the thermodynamic stability of the k phase is the lowest in the CeO2-ZrO2 system. The k phase forms metastable solid solutions with the pyrochlore phase; it was virtually stable around 1123 K. Two kinds of tetragonal phases exist, tet. (high temp.) and tet. (low temp.), which may correspond to t＇ and t*, respectively. A change in p(o2) corresponding to the phase transitions : k --> t＇ and t＇ reversible arrow t* was observed. The standard Gibbs energies of formation, Delta G degrees, of k(CeZrO4) and t*(CeZrO4) for the reaction : 1/2Ce(2)Zr(2)O(7) + 1/4 O-2 --> CeZrO4 were evaluated from the emf data.