Tetraammonium cadmium dihydrogenselenite(IV) diselenate(VI), (NH(4))(4)Cd(HSe(IV)O(3))(2)(Se(VI)O(4))(2), is the third example of a compound containing both hydrogen selenite and selenate anions, and has a new structure type. It contains kröhnkite-like heteropolyhedral chains in which CdO(6) octahedra are linked via bridging HSeO(3) groups, having their remaining two trans apices decorated by SeO(4) groups. The charge-balancing NH(4) groups are involved in weak hydrogen bonding, whereas the H atom of the HSeO(3) group provides a strong hydrogen bond [O...O = 2.614 (5) A]. The average Cd-O bond length is 2.298 A. All atoms are on general positions except Cd (on -1). Relations to the kröhnkite-type compounds Na(2)Mg(SO(3)).2H(2)O, Ba(2)CoCl(2)(SeO(3))(2) and Ba(2)Ca(HPO(4))(2)(H(2)PO(4))(2), and to the mineral curetonite are discussed. Unit-cell data are given for an isotypic Mn(II) analogue.