Two new ternary vanadyl(IV) orthophosphates, RbVOPO 4 and CsVOPO 4, have been prepared and their structures have been determined from single-crystal X-ray diffraction data. They crystallize in the orthorhombic space group P2 12 12 1 with a = 7.8669(9), b = 7.5848(8), c = 8.3771(10) Ă
, V = 499.8(1) Ă
3, Z = 4, and R = 0.029 for RbVOPO 4, and a = 7.3665(15), b = 7.6146(15), c = 9.8035(17) Ă
, V = 549.9(2) Ă
3, Z = 4, and R = 0.029 for CsVOPO 4. These two compounds consist of intersecting tunnels running along each axis, and the alkali metal cations are located at the intersection of these tunnels. Unlike other AM OPO 4 structures ( A = alkali metals; M = Ti, V), the frameworks of the title compounds are built up from discrete VO 5 pyramids and PO 4 tetrahedra. Each VO 5 pyramid shares its four basal oxygen atom vertices with three different PO 4 tetrahedra. One of the phosphate groups is coordinated to the V atom as a bidentale ligand. These two closely related structures are not isomorphous, differing in the orientation of vanadyl oxygens.