The equilibrium p(O 2)-T-δ diagrams of perovskite-type La 1 - xSr xCoO 3 - δ (x = 0.3-0.7), collected at 873-1223 K in the oxygen partial pressure range 10 - 5-1 atm by coulometric titration and thermogravimetric analysis, were analyzed in order to appraise the effects of the point-defect interactions. The nonstoichiometry variations were adequately described combining the rigid-band approach for delocalized holes and the pair-cluster formation reaction involving oxygen vacancies and Co 2+ cations, whilst coulombic repulsion between the positively charged vacancies can be neglected. The resultant relationships between the oxygen chemical potential and mobile vacancy concentration were used for numerical regression analysis of the steady-state oxygen permeation through dense La 1 - xSr xCoO 3 - δ membranes, affected by the surface exchange kinetics when Sr 2+ content is higher than 40-50%. The calculated ionic conductivity is strongly influenced by the defect association processes, and decreases with decreasing concentration of the mobile vacancies as clustering starts to prevail on reduction. The Mössbauer spectroscopy studies of La 1 - xSr xCoO 3 - δ, doped with 1 mol% 57Fe isotope and moderately reduced at p(O 2) ≈ 10 -5 atm, show no long-range vacancy ordering at x ≤ 0.5. © 2010 Elsevier B.V. All rights reserved.