Zippeite was synthezised by adjusting a UO2SO4 solution containing K2SO4 to a pH of 3.6 by means of KOH and keeping it for 75 hours at 150 degrees C and an approximate pressure of 3.5 MPa The crystals are yellow and well-formed. Chemical analysis gives K(UO2)(2)(SO4)(OH)(3) . H2O as the composition. The strongest lines of the X-ray pattern correspond to d values of 7.06, 3.51, 3.14, 2.86, and 2.65 Angstrom. The pattern is identical to the pattern of natural zippeite. Single-crystal X-ray studies revealed the composition K(UO2)(2)SO4(OH)(3) . H2O. The crystals are monoclinic, space group C2/c, with a 8.755(3), b 13.987(7), c 17.730(7) Angstrom, beta 104.13(3)degrees, and Z=8. The density D-m is 4.8, and D-x is 4.7 g/cm(3). The crystal structure was solved by Patterson methods. The structure has been refined to an unweighted residual of 0.053. Zippeite possesses a layer structure parallel to (010). UO4(OH)(3) pentagonal bipyramids are the building blocks of a unique pattern of double polyhedra linked along the a axis by edge-sharing of OH-groups. The UO4(OH)(3) chains are joined by chains of SO4 polyhedra into infinite (UO2)(2)(OH)(2)SO4 sheets. Two (UO2)(2)(OH)(2)SO4 sheets sandwich planar layers of K+, OH-, and H2O at gamma approximate to 1/4 and 3/4. The crystals show a moderate fluorescence between 520 and 620 nn, with unresolved bands at room temperature, but with three distinct bands at 77 K. The infrared spectrum was recorded, and the most important bands were assigned. The optical parameters were determined. The crystals have a moderate solubility, With the solubility product found to be 10(-42.60).