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The defect structures of the LaCoO_3−δ 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, (La³⁺,Va)₁(Co²⁺,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.