Changes occurring in the chemical state, and the crystallographic and electronic structure of LaCoO3in a reducing atmosphere were determined. These determinations involved the use of thermogravimetric and calorimetric techniques, x‐ray diffraction, and magnetic property measurements. It was shown that the reduction process proceeded through the formation of a series of oxygen‐deficient structures. In the 0–3.25 wt% weight‐loss region, corresponding to the general formula LaCo1−xO3−y, where 0.0<x≤0.08 and 0.0<y≤0.5, the structure is monoclinic. In this structure, the cobalt ion exists as either high‐spin Co2+or low‐spin CoIII. At a weight loss of approximately 3.25 wt% the structure reverted to the original rhombohedral symmetry with a loss of lanthanum and cobalt. The chemical formula was again LaCoO3. Samples reduced to greater than 3.25 wt% possessed a different crystal structure than was previously observed.