A new alkali uranyl vanadate, Rb 6 U 5 V 2 O 2 3 has been synthesized by solid-state reactions. According to synthesis method, two allotropic varieties have been found and their structures determined from single-crystal X-ray diffraction data. Both compounds crystallize in the monoclinic system with space groups C2/c and P2 1 /n for α and β varieties, respectively. α-Rb 6 U 5 V 2 O 2 3 unit-cell parameters are a=24.887(8)Å, b=7.099(2)Å, c=14.376(4)Å, β=103.92(1) o , V=2465.2(6)Å 3 , Z=4, ρ m e s =5.86(2)gcm - 3 , ρ c a l =5.85(1)gcm - 3 and for β-Rb 6 U 5 V 2 O 2 3 a=7.1635(9)Å, b=14.079(2)Å, c=24.965(4)Å, β=90.23(1) o , V=2517.8(6)Å 3 , Z=4, ρ m e s =5.79(2)gcm - 3 , ρ c a l =5.73(1)gcm - 3 . A full-matrix least-squares refinement yielded, respectively, R=0.032 and 0.041 for 1307 and 2276 independent reflections with I>2σ collected on a Smart diffractometer (MoKα radiation). Both structures are characterized by [(UO 2 ) 5 (VO 4 ) 2 O 5 ] 6 - layers which are flat and parallel to the (101) plane in α variety, corrugated and parallel to the (010) plane in the β variety. The layers are built up from VO 4 tetrahedra and UO 7 pentagonal bipyramids. In β variety, UO 6 distorted octahedra can also be considered. The UO 7 pentagonal bipyramids are associated by sharing opposite equatorial edges to form zig-zag infinite chains (UO 5 ) ~ parallel to the b-axis in α variety and to the a-axis in β variety, respectively. These chains are linked together on one side by VO 4 tetrahedra and on other side in α variety by UO 7 pentagonal bipyramids sharing corner and opposite edges and in β variety by UO 6 octahedra sharing opposite edges. Both structures differ by several characteristics. In α variety, the layers are flat according to the rows of VO 4 tetrahedra parallel to the b-axis whose apical oxygen atoms are alternatively pointing on the both sides in each row whereas in β variety, they are all pointing in the same side in a row parallel to a and in opposite side in the next, yielding to corrugated layers.