Two new alkali uranyl oxychloro vanadates M 7 (UO 2 ) 8 (VO 4 ) 2 O 8 Cl with M=Rb, Cs, have been synthesized by solid-state reactions and their structures determined from single-crystal X-ray diffraction data. They crystallize in the orthorhombic system with space groups Pmcn and Pmmn, respectively. The a and b unit cell parameters are almost identical in both compounds while the c parameter in the Rb compound is doubled: Rb-a=21.427(5)Å, b=11.814(3)Å, c=14.203(3)Å, V=3595.1(1)Å 3 , Z=4, ρ m e s =5.93(2)g/cm 3 , ρ c a l =5.82(1)g/cm 3 ; Cs-a=21.458(3)Å, b=11.773(2)Å, c=7.495(1)Å, V=1893.6(5)Å 3 , Z=2, ρ m e s =6.09(2)g/cm 3 , ρ c a l =6.11(1)g/cm 3 . A full-matrix least-squares refinement yielded R 1 =0.0221, wR 2 =0.0562 for 2675 independent reflections and R 1 =0.0386, wR 2 =0.1042 for 2446 independent reflections, for the Rb and Cs compounds, respectively. Data were collected with Mo(Kα) radiation and a charge coupled device (CCD) detector of a Bruker diffractometer. Both structures are characterized by [(UO 2 ) 8 (VO 4 ) 2 O 8 Cl] n 7 n - layers parallel to the (001) plane. The layers are built up from VO 4 tetrahedra, UO 7 and UO 6 Cl pentagonal bipyramids, and UO 6 distorded octahedra. The UO 7 and UO 6 Cl pentagonal bipyramids are associated by sharing opposite equatorial edges to form infinite chains (UO 5 -UO 4 Cl-UO 5 ) n parallel to the a axis. These chains are linked together by VO 4 tetrahedra, UO 6 octahedra, UO 7 corner sharing and UO 6 Cl, Cl sharing. Both structures differ simply by the symmetry of the layers. The unit cell contains one centrosymmetric layer in the Cs compound, whereas in the two-layer unit cell of the Rb compound, two non-centrosymmetric consecutive layers are related by an inversion center. The layers appear to be held together by the alkali ions. The mobility of the M + ions within the interlayer space in M 7 (UO 2 ) 8 (VO 4 ) 2 O 8 Cl and carnotite analog compounds is compared.