[en] A new vanadium(III) phosphate, Ba[V^III₂(HPO₄)₄](H₂O), has been synthesized by hydrothermal methods, and its structure solved by single-crystal X-ray diffraction. This vanadium phosphate crystallizes in the monoclinic system, space group P2₁. The cell parameters are a=9.441(2), b=7.913(2), c=9.521(2) A; β=117.91(2) ; V=628.5(3) A³; Z=2; D_calc=3.387 mg mm^-3; R=0.055 and R_w=0.065 for 1885 reflections with F>6.0σ(F). The three-dimensional [V₂(HPO₄)₄(H₂O)]^2- lattice can be described as being built up of chains of eight-membered rings along the c axis, formed by corner-shared VO₆ octahedra and HPO₄ tetrahedra (-V-O-P-O-), which are crosslinked via interchain V-O-P bonding to produce a three-dimensional lattice. The crosslinking interaction produces 16-membered rings. Contrary to other low-valent vanadium phosphates, cavities are found in the solid, rather than tunnels. The Ba²⁺ and H₂O are located in these cavities. (orig.)