The basic steps in the reaction of Nd 2 (SO 4 ) 3 with CS 2 were studied by thermogravimetric techniques to elucidate the reaction mechanism of the formation of Nd 2 S 3 . The Nd 2 (SO 4 ) 3 first decomposes to Nd 2 O 2 SO 4 in the range 500-650°C, depending on the heating rate. Nd 2 O 2 SO 4 is further deoxygenated to Nd 2 O 2 S, which forms a disulfide phase at lower temperatures (500-600°C) by reaction with CS 2 . Neodymium disulfide, which exhibits a wide range of non-stoichiometry (NdS 1 . 8 7 6 -NdS 2 ), decomposes into Nd 2 S 3 above 600°C. At lower heating rates (1 and 5°C min - 1 ), orthorhombic Nd 2 S 3 is formed in a single phase on heating to 1100°C. In contrast, non-stoichiometric cubic Nd 2 S 3 - x is formed together with orthorhombic Nd 2 S 3 when the heating rate is as high as 10°C min - 1 .