Ce1-xTbxO2-delta solid solutions (x = 0.3, 0.4, and 0.5) were synthesized by a coprecipitation method, using ammonia. The formation process of the solid solutions was studied as a function of temperature up to 1200 degrees C by X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry. The results showed that chemically homogeneous, oxidic fee structures were formed. The large increase (similar to 1.5%) of the fee unit-cell parameter on increase of temperature in ambients of oxygen or nitrogen could be correlated with changes in the oxygen stoichiometry (approximate to 3.5%) that have led to a reversible change in the valence state from (TV) to (III) of a fraction of the Ce and Tb ions. The fee structure was chemically and structurally stable to at least 1200 degrees C.