For LaMnO3+delta with oxygen-excess nonstoichiometry, in situ X-ray diffractometry was performed under high temperatures and precisely controlled oxygen partial pressures, P-O2. A rhombohedral phase was observed under most experimental conditions. With temperature rising, the lattice volume increased linearly in a low temperature region; the coefficient of linear thermal expansion (CTE) was 1.06 x 10(-5) K-1. In a high temperature region, the temperature dependence of the lattice volume upwardly deviated from the linearity, indicating that the unit cell expanded upon decrease in excess oxygen. The nonstoichiometry coefficient of the linear isothermal expansion was calculated at 5 x 10(-2) using literature data on delta(T, P-O2). The rhombohedral distortion escalated with temperature and P-O2 decreasing. The dependence of cell volume and distortion on nonstoichiometry is well interpreted as the effect of variation in the ionic radius and tolerance factor. (C) 2004 Elsevier B.V. All rights reserved.