A series of lanthanide and yttrium propylenediphosphonates has been prepared hydrothermally (210°C, 4 days) by action of propylenediphosphonic acid on the metal chlorides. The crystal structure of the gadolinium compound Gd ^III H[O ₃ P(CH ₂ ) ₃ PO ₃ ] has been determined ab initio from X-ray powder diffraction data and refined by the Rietveld method. The compound crystallizes in the monoclinic space group C2/m (no. 12) with a=8.2141(3) Å, b=18.9644(8) Å, c=5.2622(2) Å, β=111.999(2) ° and Z=4. The final agreement factors are R _p =0.113, R _wp =0.142, Bragg R=0.050, R _F =0.034 and χ ² =1.91. In the three-dimensional structure, the gadolinium atoms are eight-coordinated. The framework consists of inorganic Gd-P-O sheets joined by organic groups with an interlayer spacing of 9.58 Å. The entire series of the lanthanide elements and yttrium give isotypic structures. Attempts with ethylenediphosphonic acid and gadolinium led to the analogous compound Gd ^III H[O ₃ P(CH ₂ ) ₂ PO ₃ ] which crystallizes in the monoclinic space group P2 ₁ /c (no. 14) with cell parameters a=5.2918(9), b=15.975(3), c=8.338(1) Å, β=111.491(6), Z=4 (final agreement factors R _p =0.078, R _wp =0.105, Bragg R=0.034, R _F =0.026 and χ ² =1.50), and with an interlamellar distance of d=7.99 Å. Moreover, the action of methylenediphosphonic acid on La, Ce, Pr and Nd chloride led to a similar structure with a shorter interlamellar distance, d=7.03 Å for Pr ^III H[O ₃ P(CH ₂ )PO ₃ ] (space group C2/m, with a=8.3271(4), b=14.0645(7), c=5.3489(3) Å, β=111.433(2)°, Z=4, and final agreement factors R _p =0.092, R _wp =0.121, Bragg R=0.059, R _F =0.035 and χ ² =1.98).