Thermal, mechanical, and phase stability of LaCoO3 perovskite in air and 4% H-2/96% Ar reducing atmosphere have been studied by thermal mechanical analysis (TMA), high temperature microhardness, and high temperature/room temperature X-ray diffraction. The thermal behavior of LaCoO3 in air exhibits a non-linear expansion in the 100-400 degrees C temperature range. A significant increase of coefficient of thermal expansion (CTE) measured in air both during heating and cooling experiments occurs in the 200-250 degrees C temperature range, corresponding to a known spin state transition. LaCoO3 is found to be highly unstable in a reducing atmosphere. In case where LaCoO3 was present as a powder, where surface reduction mechanism would prevail, the reduction starts as earlier as 375 degrees C with a formation of the metallic Co and La2O3 at 600 degrees C. In the bulk form, LaCoO3 undergoes a series of expansion and contractions due to phase transformations beginning around 500 degrees C with very intensive chemical/phase changes at 800 degrees C and above. These expansions and contractions are directly related to the formation of La3Co3O8, La2CoO4, La4Co3O10, La2O3, CoO, and other Co compounds in the reducing atmosphere. Although LaCoO3 is a good ionic and electronic conductor and catalyst, its high thermal expansion as well as structural, mechanical, and phase instability in reducing environments present a serious restriction for its application in solid oxide fuel cells, sensors or gas separation membranes. (C) 2008 Elsevier B.V. All rights reserved.