The stability of the La 3 . 5 Ru 4 O 1 3 and La 2 RuO 5 compounds in the La-Ru-O system in various atmospheres and various temperature ranges was investigated by thermal analysis, X-ray diffraction analysis and electron microscopy. The La 3 . 5 Ru 4 O 1 3 compound is stable in oxidizing and neutral atmospheres (N 2 with 10ppm O 2 ), while La 2 RuO 5 is partially reduced in a neutral atmosphere to form La 2 RuO 4 . 6 . In a reducing atmosphere both compounds decompose into metallic Ru and La 2 O 3 . The thermal expansion coefficients of La 2 RuO 5 and La 3 . 5 Ru 4 O 1 3 at 800 o C are 11.2 x 10 - 6 K - 1 and 9.3 x 10 - 6 K - 1 , respectively. The specific electrical resistivity for La 3 . 5 Ru 4 O 1 3 is relatively independent of temperature and is 2 x 10 - 2 Ωcm at 800 o C, while for La 2 RuO 5 it decreases with increasing temperature and is 1Ωcm at 800 o C.