A new caesium uranyl molybdate belonging to the M 6 U 2 Mo 4 O 21 family has been synthesized by solid-state reaction and its structure determined from single-crystal X-ray diffraction data. Contrary to the other alkali uranyl molybdates of this family (A=Na, K, Rb) where molybdenum atoms adopt only tetrahedral coordination and which can be formulated A 6 [(UO 2 ) 2 O(MoO 4 ) 4 ], the caesium compound Cs 6 U 2 Mo 4 O 21 should be written Cs 6 [(UO 2 ) 2 (MoO 4 ) 3 (MoO 5 )] with molybdenum atoms in tetrahedral and square pyramidal environments. Cs 6 [(UO 2 ) 2 (MoO 4 ) 3 (MoO 5 )] crystallizes in the triclinic symmetry with space group P1̄ and a=10.4275(14)Å, b=15.075(2)Å, c=17.806(2)Å, α=70.72(1)°, β=80.38(1)° and γ=86.39(1)°, V=2604.7(6)Å 3 , Z=4, ρ mes =5.02(2)g/cm 3 and ρ cal =5.08(3)g/cm 3 . A full-matrix least-squares refinement on the basis of F 2 yielded R 1 =0.0464 and wR 2 =0.0950 for 596 parameters with 6964 independent reflections with I≥2σ(I) collected on a BRUKER AXS diffractometer with Mo(Kα) radiation and a CCD detector. The crystal structure of Cs compound is characterized by ∞ 1 [(UO 2 ) 2 (MoO 4 ) 3 (MoO 5 )] 6− parallels chains built from U 2 O 13 dimeric units, MoO 4 tetrahedra and MoO 5 square pyramids, whereas, Na, K and Rb compounds are characterized by ∞ 1 [(UO 2 ) 2 O(MoO 4 ) 4 ] 6− parallel chains formulated simply of U 2 O 13 units and MoO 4 tetrahedra.Infrared spectroscopy measurements using powdered samples synthesized by solid-state reaction, confirm the structural results. The thermal stability and the electrical conductivity are also studied. The four compounds decompose at low temperature (between 540 and 610°C).