Compounds K[UO(CHO)] · HO ( I) and Rb[UO(CHO)] · HO ( II) are synthesized and their crystal structures are determined by X-ray diffraction. The compounds crystallize in the monoclinic crystal system; for I, a = 7.1700(2) Å, b =12.3061(3) Å, c = 14.3080(4) Å, β = 95.831(2)°, space group P2/ n, Z = 4, and R = 0.0275; for II, a = 7.1197(2) Å, b = 12.6433(4) Å, c = 14.6729(6) Å, β = 96.353(2)°, space group P2/ n, Z = 4, and R = 0.0328. It is found that I and II are isostructural. The main structural units of the crystals are the [UO(CHO)] chains, which belong to the AT B ( A = UO, T, and B = CHO) crystal chemical group of uranyl complexes. The chains and alkali metal ions R ( R = K or Rb) are connected by electrostatic interactions and hydrogen bonds. Some specific structural features of [UO(CHO4)] complex groups are discussed.