Studtite, UO4·4H2O, and metastudtite, UO4·2H2O, are the only minerals thought to contain peroxide. Determination of the structure of studtite has shown it to contain peroxide, with the structural formula [(UO2)(O2)(H2O)2](H2O)2. The structure is monoclinic, space group C2/c, a = 14.068(6), b = 6.721(3), c = 8.428(4) Å, β = 123.356(6)°, V = 665.6(3) Å3, Z = 4. It was refined on the basis of F2 for 1398 unique reflections collected using MoKα X-radiation and a CCD-based detector to R1 = 3.66%, calculated for the 716 unique observed reflections (|Fo| ≥ 4σF). The structure of studtite contains one symmetrically distinct U6+ cation and four O atoms, two of which occur as H2O groups. The O-O bond-length in the peroxide group is 1.46(1) Å. The U6+ cation occurs as part of a linear (UO2)2+ uranyl ion, and each U6+ cation is bonded to six additional O atoms, two of which are H2O groups, and four of which are O atoms of peroxide groups. The O-O bonds of two peroxide groups constitute two equatorial edges of each distorted uranyl hexagonal bipyramid. Uranyl polyhedra are polymerized into chains extending along  by sharing peroxide groups. Chains are linked by H bonds extending to and from an interstitial H2O group. It is proposed that studtite forms by incorporating peroxide created by alpha-radiolysis of water, and that radiation is necessary for its formation in nature.