Cerium sulfides were prepared by high-energy milling of cerium and sulfur. The reaction kinetics were observed by monitoring temperature and pressure in situ. For the first time, it is demonstrated that cerium monosulfide, which has historically been produced only at very high temperatures using both toxic and flammable gases, can be produced from a stoichiometric mixture of the elemental powders at near room temperature. It is shown that the monosulfide is produced through a mechanochemically induced self-propagating reaction of intermediate compounds.