Abstract. Thorium peroxide was precipitated from thorium nitrate solutions containing varying amounts of sulfate ion and of hydrogen ion. The washed solids were analyzed both wet and dry. Analyses were made for thorium, peroxide oxygen, sulfate, nitrate, and water contents. X-ray powder photographs of the dried samples by W.H. Zachariasen showed the presence of only two phases. When precipitated from high sulfuric acid, the solid phase was isomorphous with Th(OO)SO4.3H20. More weakly acid solutions whose sulfate content was varied over wide limits [SO4--/Th(IV) (in solution before precipitation) = 0.005 to 67.0] yielded a solid, isomorphous with thorium peroxide nitrate and containing 3.0 to 3.8 peroxide oxygen atoms for each thorium atom. The sulfate content of this latter phase varied continuously between the limits SO4--/Th(IV) = 0.34 to 0.01 with nitrate ion present when the sulfate content was very low. This continuous variation in composition is in agreement with the previously proposed structure of this phase.