Abstract Solid state equilibrium relations were studied in the region of the La-Mn-O system bounded by LaMnO 3, MnO and La 2O 3. In the temperature range 900–1380°C the defective perovskite LaMnO 3−λ coexists directly in equilibrium with lanthanum oxide and manganous oxide. Above 1380°C an intermediate phase with the formula La 2MnO 4.15 appears. This phase has a structure of the K 2NiF 4 type with cation vacancies. The number of anion vacancies in LaMnO 3−λ increases with increasing temperature; at the limiting composition the value of λ changes from 0.015 at 900°C to 0.095 at 1450°C. The behaviour of the oxygen pressure of the LaMnO 3−λ-La 2O 3-MnO equilibrium system agrees with this variation in the composition of the LaMnO 3−λ phase.