The defect structures of the LaCoO3-delta perovskite are analyzed through thermodynamic modeling by means of the CALPHAD technique. It is discovered that charge disproportionation exists for Co3+ in order to describe the relationship between the oxygen partial pressure and the oxygen deficiency. Based on available experimental data, a compound-energy model, (La (3) (+),Va) (1) (Co (2+),Co3+,Co4+, Va) (1) (O2- Va)(3), where Va stands for vacancy, is proposed and model parameters evaluated. It was consequently predicted from the thermodynamic model that the amounts of Co2+ and Co4+- are about 20% each when there is no oxygen deficiency. (c) 2007 Elsevier BY. All rights reserved.