Hydrous vanadium (IV, V) oxides containing K or Rb were synthesized by the hydrothermal method from VOSO4-M(2)SO(4) (M = K, Rb) solutions. They were found to be isomorphous with previously reported Ba0.4V3O8(VO)(0.4). nH(2)O, whose structure consists of V3O8 layers which are bridged by V-O units (denoted by (VO)(0.4) in the formula) forming a tunnel-like opening to accommodate hydrous Ba ions. The isomorphous compounds can be formulated by M(x)V(3)O(8)(VO)(y) . nH(2)O (M = K, Rb, Ba; n = 0.6-0.7) where x and y show similar values of less than 0.5 as is accounted for by the crystal structure. The formation of this phase seems to be governed by the size of M cation whose ionic radius should lie in a limited range of 1.35 to 1.52 Angstrom. The V3O8(VO)(y) framework structure of a bronze-like type is compared with the V2O5 framework of beta-M(x)V(2)O(5) (beta bronze) and the V3O8 framework of H2V3O8.