Single crystals of hydrated vanadium oxides with layered structures containing divalent Ca or Ni were hydrothermally grown, and their structural characterization was focused on the interlayer sites. Both compounds are formulated by M 0.25V 2O 5·H 2O, M=Ca, Ni and crystallize in the monoclinic system C2/ mwith Z=4: a=11.692(2), b=3.564(1), c=10.986(2) Å, b=105.42(1)° for M=Ca; a=11.756(1), b=3.649(1), c=10.364(2) Å, β=95.03(1)° for M=Ni. The structures consist of δ-type V 2O 5layers stacking along the caxis and interlayer hydrated M 2+ions; the δrefers to the V 2O 5layer of δ-Ag 0.68V 2O 5after which the compounds are designated δ- M 0.25V 2O 5·H 2O. The Matoms partially occupy interlayer sites of the 2 cposition for M=Ni and the 4 iposition for M=Ca, to each of which four water molecules (O w) are attached forming MO w4rectangles. Mand O wsite occupancies indicate that the MO w4rectangles are distributed so that they do not interact with each other, and the above stoichiometric formula correspond to the maximum occupancies. Bonding to three apical oxygens of the V 2O 5layers, the Ca atom forms a CaO 7polyhedron, and to two apical oxygens the Ni atom forms a NiO 6octahedron, due to the Ca 2+ion being larger than that of Ni 2+ion. This is the first single-crystal study on the structures of hydrated δ-type vanadium oxide bronzes.