Ferroselite, FeSe 2 , is commonly associated with sandstone-type uranium deposits. Although it has been previously synthesized, the conditions of synthesis were not applicable to the natural system, and the literature did not provide any information on the formation of natural ferroselite. The U. S. Geological Survey's Ambrosia Lake study, however, was concerned with the formation of ferroselite under more natural conditions. Thus, in this work, the synthesis of ferroselite was restricted to conditions more nearly approaching the conditions of the formation of sandstone-type uranium deposits--a low temperature and an aqueous environment. A solution of sodium selenosulfate, Na 2 SSeO 3 , stood in contact with freshly precipitated iron sulfide, FeS, for 10 days under an inert atmosphere at 80 degrees C. The final product was identified by its diffraction pattern and consisted of FeSe 2 mixed with about 10 percent FeS.