The structure of KSmHP 3 O 1 0 has been determined by a single crystal X-ray analysis and has been solved by using 3299 independent reflections with a final R value of 0.019. This condensed phosphate crystallizes in the triclinic space group P1 with unit cell parameters a=6.956(2) 9, b=7.711(1) 9, c=8.570(2) 9, α=106.64(2) o , β=106.63(2) o , γ=82.47(3) o , V=421.4(2) 9 3 and D c a l =3.494 g cm - 3 for Z=2. The structure of NH 4 SmHP 3 O 1 0 -ammonium samarium hydrogen triphosphate has been determined from conventional X-ray powder diffraction data using the Rietveld method, with the isostructural compounds KSmHP 3 O 1 0 and NH 4 BiHP 3 O 1 0 as starting model. The cell is triclinic (space group P1 Z=2), with a=6.9782(2) 9, b=7.7578(2) 9, c=8.6230(3) 9, α=106.354(2) o , β=105.961(2) o , γ=82.471(2) o , V=429.93(7) 9 3 and D c a l =3.26 g cm - 3 . Refinement of 83 parameters by the Rietveld method, using 2547 reflections, leads to cR w p =0.152, cR p =0.117 and R B =0.0437. The unit cell contains two P 3 O 1 0 5 - groups linked with OH...O hydrogen bonding along the c axis. The samarium atoms in a eightfold co-ordination build infinite chains of edge-sharing SmO 8 polyhedra running parallel to the a axis. The cations in this structure (K + and NH 4 + ) are located between chains formed of (HP 3 O 1 0 4 - ) to form two dimensional zig zag sheets. The two polyhedra (SmO 8 and KO 1 0 or (NH 4 )O 1 0 ) are connected by P 3 O 1 0 5 - to built a three dimensional network.