Oxygen non-stoichiometry ofLa(0.8)Sr(0.2)Sc(y)Mn(1-y)O(3 +/-delta) (y = 0.2, 0.3, 0.4) oxide was studied by coulometric titration as a function of oxygen partial pressure, p(O-2), and temperature in a range of 923-1023 K. Depending on the Sc doping amount, p(O-2), and temperature, oxygen non-stoichiometry varies significantly. Under a reducing condition, La0.8Sr0.2ScyMn1-yO3 +/-delta shows both oxygen excess and oxygen deficient compositions. At the higher p(O-2) region, the oxygen excess composition is due to metal ion vacancies, whereas in the lower p(O-2) region, the oxygen deficient composition is due to the formation of oxygen vacancies. The experimental data were analyzed by a random defect model. Partial molar enthalpy and partial molar entropy of oxygen vacancy formation are calculated using the Gibbs-Helmholtz equation from the non-stoichiometric data. The electrical conductivity was measured as the function of the oxygen partial pressure and temperature. In the lower p(O-2) region, electrical conductivity strongly depends on the oxygen non-stoichiometry. (C) 2012 Elsevier B.V. All rights reserved.