Reduction of pure and doped CeO2 to CeO2-x is associated with an expansion. The behaviour of pure CeO2, Ce0.9Ca0.1O1.9, Ce0.8Ca0.2O1.8 and Ce0.6Gd0.4O1.8 at 1000-degrees-C in the oxygen partial pressure range from 0.21 atm to 10(-18) atm is described. The results show that even though the nominal concentration of oxide vacancies are equal in the two mostly doped cerias, the CaO-doped one expands much more than the Gd2O3-doped one. The max. expansion of CeO2 was about 2.2%, of Ce0.8Ca0.2O1.8 1.5%, and Ce0.6Gd0.4O1.8 0.8%. The results are compared with high temperature XRD data from literature.