Lanthanum manganates with the perovskite structure are generating considerable interest due both to their use as solid oxide fuel cells (SOFC) cathode material and due to their interesting magnetoresistive properties, making them a candidate material for reading heads in magnetic storage devices. For both these applications, the defect chemistry of the perovskites is of utmost importance as this largely determines both the electrical and magnetic properties of the material. We model the Gibbs energy of the perovskite using the compound energy formalism under the assumption that Mn2+ or Mn3+ form antisite defects on the La-sublattice and that no antisite defects are formed. We find that only if antisite defects are included, can the experimental data on oxygen nonstoichiometry of cation nonstoichiometric perovskites from literature be reproduced. However. the question whether the antisite Mn assumes the valency Mn2+ or Mn3+ cannot be answered with this modeling method, as both versions reproduce the defect chemistry equally well. (C) 2004 Elsevier B.V. All rights reserved.