Two new alkali uranyl vanadates, M 6 (UO 2 ) 5 (VO 4 ) 2 O 5 with M β«Ψβ¬ Na, K, have been synthesized by solid state reactions and their structures determined from single-crystal X-ray di4raction data. They crystallize in the monoclinic system with space group P2 1 /c. Unit cell b, c, and parameters are almost identical in both compounds while the a parameter in the Na compound is doubled: Na0a β«Ψβ¬ 12.584( 1) A s , b β«Ψβ¬ 24.360(2) A s , c β«Ψβ¬ 7.050(1) A s , β«Ψβ¬ 100.61(1)3, V β«Ψβ¬ 2124.2 A s , Z β«Ψβ¬ 4, D mes β«Ψβ¬ 5.60(2) g/cm 3 , D cal β«Ψβ¬ 5.63 g/cm 3 ; K0a β«Ψβ¬ 6.856(1) A s , b β«Ψβ¬ 24.797(3) A s , c β«Ψβ¬ 7.135(1) A s , β«Ψβ¬ 98.79(8)3, V β«Ψβ¬ 1198.8 A s , Z β«Ψβ¬ 2, D mes β«Ψβ¬ 5.25(2) g/cm 3 , D cal β«Ψβ¬ 5.25 g/cm 3 . A full-matrix least-squares re5nement yielded, respectively, R β«Ψβ¬ 0.039, R w β«Ψβ¬ 0.043 and R β«Ψβ¬ 0.037; R w β«Ψβ¬ 0.046 for 3293 and 2096 independent re6ections with I > 3 collected on a Philips PW 1100 di4ractometer (MoK radiation). Both structures are characterized by [(UO 2 ) 5 (VO 4 ) 2 O 5 x] 6nΨ n corrugated layers parallel to the (100) plane. The layers are built up from VO 4 tetrahedra, UO 7 pentagonal bipyramids, and UO 6 distorted octahedra. The UO 7 pentagonal bipyramids are associated by sharing opposite equatorial edges to form zig-zag in5nite chains (UO 5 ) n parallel to the c axis. These chains are linked together on one side by VO 4 tetrahedra and on other side by UO 6 octahedra and UO 7 corner sharing. Both structures di4er simply by the symmetry of the layers. The unit cell contains one centrosymmetric layer in the K compound, whereas in the two-layer unit cell of the Na compound two noncentrosymmetric consecutive layers are related by an inversion center. The layers appear to be held together by the alkali ions. The mobility of the M Ψ ions within the interlayer space in M 6 (UO 2 ) 5 (VO 4 ) 2 O 5 and carnotite analog compounds is compared.