The thermal decomposition of hydrated thorium oxalate was followed by determining weight loss, change in electrical conductivity, and chemical composition of products. At temperatures below 200°C. the weight-loss curve indicated the existence of a tri-and a dihydrate and possibly a pentahydrate of thorium oxalate as stages m the dehydration. At higher temperatures dehydration was accompanied by loss of CO and CO,.Almost all the (C,O,)" decomposed sharply over a small temperature range near 300 OC. Although some carbonate was formed, it did not exceed a concentration of 0.076 mole per mole of total thorium.According to chemical analysis the residue after heating a t 360 O C . was Tho, with a trace of carbonate and water. X-ray analysis, however, showed it to be a mixture of Tho, and a second substance whose identity is unknown.I n the electron microscope the hydrated oxalate and the thoria produced from it were httle different in appearance. Both appeared as relic crystals which were square prisms about l y across and 0 . 2 y thick, with a characteristic central aperture which exposed a large area of surface.* Total thorium was known from the weight and composition of the sample before heating.