For application of Ln---Si---O---N (Ln=Y, Gd and La) oxynitride materials, e.g. as host-lattices for lamp phosphors, oxidation resistance up to about 600 °C in air is a prerequisite. In this study we prepared Ln---Si---O---N (Ln=Y, Gd and La) powders by solid state reaction and observed via TGA/DTA-experiments that most compounds are oxidation resistant up to 600 °C in air. The stability in air at high temperatures increases going from Ln5(SiO4)3N, Ln4Si2O7N2, LnSiO2N, Ln2Si3O3N4 to Ln3Si8O4N11. This is explained by an increasing cross-linking between the silicon---oxygen---nitrogen tetrahedra in this sequence. For the lattices with less cross-linking between the silicon---oxygen---nitrogen tetrahedra we observed that the oxidation resistance decreases slightly going from Y and Gd to La. For these lattices, also, an additional weight gain was observed during the oxidation reaction, which was higher than expected for complete oxidation. The additional weight gain was ascribed to an intermediate phase in which nitrogen retention takes place.