The transition metal, alkali metal, and main group uranyl selenites, Ag-2(UO2)(SeO3()2) (1), K[(UO2)(HSeO3)(SeO3)] (2), Rb[(UO2)(HSeO3)(SeO3)] (3), CS[(UO2)(HSeO3)(SeO3)] (4), Tl[(UO2)(HSeO3)(SeO3)] (5), and Pb(UO2)(SeO3)(2) (6), have been prepared from the hydrothermal reactions of AgNO3, KCI, RbCl, CsCl, TICI, or Pb(NO3)(2) with UO3 and SeO2 at 180 degreesC for 3 d. The structures of 1-5 contain similar (2)(infinity)[(UO2)(SeO3)(2)](2-) sheets constructed from pentagonal bipyramidal UO7 units that are joined by bridging SeO32- anions. In 1, the selenite oxo ligands that are not utilized within the layers coordinate the Ag+ cations to create a three-dimensional network structure. In 2-5, half of the selenite ligands are monoprotonated to yield a layer composition of (2)(infinity)[(UO2)(HSeO3)(SeO3)](1-), and coordination of the K+, Rb+, Cs+, and TI+ cations occurs through long ionic contacts. The structure of 6 contains a uranyl selenite layered substructure that differs substantially from those in 1-5 because the selenite anions adopt both bridging and chelating binding modes to the uranyl centers. Furthermore, the Pb2+ cations form strong covalent bonds with these anions creating a three-dimensional framework. These cations occur as distorted square pyramidal PbO5 units with stereochemically active lone pairs of electrons. These polyhedra align along the C-axis to create a polar structure. Second-harmonic generation (SHG) measurements revealed a response of 5x alpha-quartz for 6. The diffuse reflectance spectrum of 6 shows optical transitions at 330 and 440 nm. The trailing off of the 440 nm transition to longer wavelengths is responsible for the orange coloration of 6.