An instrument for Coulometric-titration measurements was built and used to measure the thermodynamic redox properties for a 10-wt% Cu/silica catalyst at 973 K and for two ceria-zirconia solid solutions, Ce_0.81Zr_0.19O₂ and Ce_0.25Zr_0.75O₂, between 873 and 1073 K. For Cu/silica, the equilibrium data show two well-defined steps in the oxygen isotherms, associated with equilibrium between Cu and Cu₂O and between Cu₂O and CuO; and the P(O₂) associated with these two steps are in close agreement with values expected for pure Cu. The oxidation enthalpies were similar for both Ce_0.81Zr_0.19O₂ and Ce_0.25Zr_0.75O₂, between -500 and -550 kJ/mol O₂, and independent of the extent of reduction. However, there is a step change in partial entropy of reduction for Ce_0.81Zr_0.19O₂, from ~-250 J/mol^.K to less than -100 J/mol^.K, after removal of approximately one oxygen for every two Zr⁺⁴. A model is presented which views the reduction of ceria-zirconia as removal of oxygen from "pyrochlore-like" clusters, with some of the changes in reducibility associated with the number of sites from which oxygen can be removed from such clusters.