The pentahydrated uranyl orthovanadate (UO 2 ) 3 (VO 4 ) 2 ·5H 2 O was synthesized in shaken aqueous solution at 60°C, by reaction of UO 3 or a uranyl salt (acetate or nitrate) and V 2 O 5 . Single crystals were obtained by hydrothermal reaction at 180°C. It crystallizes in an orthorhombic system, space group Cmcm, with a=17.978(2) Å, b=13.561(2) Å, c=7.163(1) Å, V=1746.3 Å 3 , Z=4, ρ cal =4.3 g·cm −3 . The structure has been refined to R=0.048 and Rw=0.047 using 1020 independent reflections with I>3σ(I) collected on a Philips PW 1100 automated diffractometer using MoKα radiation. Uranyl ions have two distinct functions. U(1)O 2+ 2 uranyl ions are surrounded in an equatorial plane by five oxygen atoms of four different VO 4 tetrahedra which form pentagonal UO 7 bipyramids. The linkage of UO 7 bipyramids and VO 4 tetrahedra leads to (UVO 6 ) ∞ layers parallel to the (100) plane. The connection between these sheets is achieved by the other U(2)O 2+ 2 uranyl ions and water molecules. Thus, the compound may be considered a uranyl vanadate of uranyl and can be formulated (UO 2 ) 0.5 [(UO 2 )(VO 4 )]·2.5H 2 O. Dehydration is reversible and leads to a new anhydrous compound (UO 2 ) 3 (VO 4 ) 2 after intermediate steps corresponding to different hydrates. Unfortunately, the anhydrous product cannot be synthesized directly from UO 3 and V 2 O 5 by solid-state reaction.