In the oxide portion of the La-Sr-O system there exist two intermediate phases, β with a certain homogeneity range centred around the stoichiometry La 4 SrO 7 and a stoichiometric phase, La 4 Sr 3 O 9 . La 2 O 3 dissolves a considerable amount of SrO, SrO only negligible amounts of La 2 O 3 . Microprobe analysis, X-ray diffraction and differential thermal analysis were used to determine the stability limits of the phases. Using the experimental data and assessed literature data, a thermodynamic description of the oxide portion of the system was optimised and the quasi-binary La 2 O 3 -SrO phase diagram calculated.