The results of oxygen nonstoichiometry, delta, as well as chemical expansion as functions of oxygen partial pressure pO(2) and temperature are presented for the perovskite-type undoped manganite LaMnO3 (+/-) (delta). The modeling of the defect structure of this perovskite is carried out in both deficit and excess oxygen regions. Within the framework of the model electrons and electron holes are assumed to be localized on manganese sites in LaMnO3 (+/-) (delta). Defect structure models proposed are shown to fit perfectly well the experimental data on oxygen nonstoichiometry of LaMnO3 (+/-) (delta). Equilibrium constants of the appropriate defect reactions are, therefore, determined. As a consequence, concentrations of all defect species defined within the frameworks of the model proposed are calculated as functions of temperature and oxygen nonstoichiometry. These dependences are employed in the chemical expansion model derived by us earlier and based on the relative change of the mean ionic radius in order to compute the LaMnO3-delta chemical expansion versus oxygen nonstoichiometry. The experimental data on the chemical expansion of LaMnO3-delta and those calculated on the basis of the model proposed are shown to coincide with each other very good. Such coincidence implies obviously that the isothermal expansion of LaMnO3-delta observed with small deviation from its stoichiometric composition with respect to oxygen at given temperature is solely caused by the mean ionic radius increase. (c) 2010 Elsevier B.V. All rights reserved.