Gadolinium ultraphosphate, GdP5O14 has been synthesized by a flux method. The room temperature structure of the title compound was solved from twinned single-crystal X-ray diffraction data in the monoclinic space group P2(1)/c with a = 8.7411(3); b = 8.8997(2); c = 12.9308(5) angstrom; beta = 90.07(2)degrees; V = 1005.93(6) angstrom(3) and Z = 4. Crystals of the title compound exhibit twinning by pseudomerohedry. The apparent symmetry is caused by twinning and by the pseudo-orthorhombic symmetry of the lattice. Due to the pseudosymmetry the space group Pnmb has been assigned and after a complete structure refinement applied to a twinned crystal, the correct space group is P2(1)/c. The atomic arrangement in the present structure is very close to the previously reported one of GdP5O14 at T = 145 K (type I) [J.M. Cole, M.R. Lees, J.A.K. Howard, R.J. Newport, G.A. Saunders, E. Schonherr, J. Solid State Chem. 150 (2000) 377], but a small deviation in the unit cell has been notified. The refinement gave R-1 = 2.1% and wR(2) = 8.4% based on 4138 independent reflections and 3619 observed, I > 2 sigma(I). The result of structural analysis establishes that the structure is built up with discrete anionic ribbons linked by the cations in a complex way. Magnetic susceptibility studies show paramagnetic behavior, following Curie's law with a magnetic moment of 7.96 mu(B) per formula unit. (c) 2007 Elsevier B.V. All rights reserved.