Nonstoichiometry of the fluorite-type oxide solid solutions Ce (1-X) YXO (2 - 0.5X -delta)(X=0.1, 0.2) was measured as a function of temperature (T=973-1373 K) and oxygen partial pressure (P(O-2)=10(-2)-10(-24) atm) by means of thermogravimetry. The result shows that the nonstoichiometry of Ce (1-X) YXO (2 - 0.5X - delta) (X= 0. 1, 0.2) is not explained by a simple point defect model, therefore, defect association models are suggested. From the calculation, it is found that (Ce'CeVOCe'(Ce))(X) is the major defect association not only in Ce (1-X) YXO2 - 0.5X - delta (X= 0. 1, 0.2), but also in pure CeO2 for nonstoichiometries less than 0.10. It is also found that the defect association (Ce'CeVOCe'(Ce))(X) is dominating at lower temperature and smaller X composition. (C) 2002 Elsevier B.V. All rights reserved.