A novel mathematical approach for defect concentration calculations proposed by Poulsen [F.W. Poulsen, Solid State Ionics, 129 (2000) 145] for oxides with rather small deviations from stoichiometry also applies to perovskite-type oxides with large ranges of oxygen nonstoichiometry. Point defect chemistry calculations have been performed for YBa2Cu3O6.5+delta for which experimental equilibrium oxygen partial pressure isotherms are well known. The model applied in this work describes experimental results very well in the Po-2 range of 10(-5) to 10 bar and for temperatures between 450 and 800 degrees C. (C) 2007 Elsevier Ltd. All rights reserved.