Partially dehydrated hydroxyapatite was characterised by X-ray powder diffraction (XRPD) using Guinier-Hagg films, diffractometer and high-resolution synchrotron data as a function of the degree of dehydration (DD). It is for the first time shown that the space group symmetry for partially dehydrated hydroxyapatite changes from hexagonal P6(3)/m to triclinic when more than ca. 35% of the structurally bound water is removed. With increasing DD the a- and b-axes decrease and the c-axis increases. At the highest DD value attained (78%) before the onset of a decomposition of the apatite, the cell parameters were determined to be a = 9.40023 (3), b = 9.39704 (3), c = 6.89967 (2) Angstrom, alpha = 90.0626 (2)degrees, beta = 89.7478 (1)degrees and gamma = 119.9971 (2)degrees. A structure refinement for this sample converged with R-F = 3.7% in space group P(1) over bar, using synchrotron data (lambda = 0.852790 Angstrom), 795 reflections in the 2 theta range 5-45 degrees and 62 positional parameters. The shifts of the atoms from their corresponding positions in hexagonal hydroxyapatite are small. A possible cause for the triclinic distortion is a tilting of the hydroxide ions in the apatite channels away from the channel axis.