The room-temperature unit-cell volumes of synthetic hydroxylapatite, Ca 5 (PO 4 ) 3 OH, fluorapatite, Ca 5 (PO 4 ) 3 (F (sub 1-x) ,OH x ) with x = 0.025, and chlorapatite, Ca 5 (PO 4 ) 3 (Cl (sub 0.7) ,OH (sub 0.3) ), have been measured by high-pressure (diamond anvil-cells) synchrotron X-ray powder diffraction to maximum pressures of 19.9 GPa, 18.3 GPa, and 51.9 GPa, respectively. Fits of the data with a second-order Birch-Murnaghan EOS (i.e. (dK/dP) (sub P = O) = 4) yield bulk moduli of Ko = 97.5 (1.8) GPa, K 0 = 97.9 (1.9) GPa and K 0 = 93.1(4.2) GPa, respectively. The room-pressure volume variation with temperature was measured on the same hydroxyl- and fluorapatite synthetic samples using a Huber Guinier camera up to 962 and 907 degrees C, respectively. For hydroxyl- and fluorapatite, the volume data were fitted to a second-order polynomial: V(T)/V 293 = 1+alpha 1 (T-293)+alpha 2 (T-293) 2 with T expressed in K leading to alpha 1OH = 2.4(+ or -0.1)X10 (super -5) K (super -1) , alpha 2OH = 2.7(+ or -0.1)X10 (super -8) K (super -2) and alpha 1F = 3.4(+ or -0.1)X10 (super -5) K (super -1) , alpha 2F = 1.6(+ or -0.1)X10 (super -8) K (super -2) , respectively. A significant increase is observed in hydroxyl-apatite thermal expansion above ca. 550 degrees C and extra reflections start to clearly appear on the X-ray film above 790 degrees C. These features are interpreted as the progressive dehydration of slightly Ca-deficient hydroxylapatite (i.e. with Ca/P<1.67). Phase relation calculations, taking these new volume data for apatite into account, show that at 1200 degrees C, in the presence of kyanite+SiO 2 , hydroxylapatite should dehydrate to form gamma -Ca 3 (PO 4 ) 2 +Ca 3 Al 2 Si 3 O 12 below 12 GPa, i.e. below the upper-pressure stability-limit of apatite that was previously determined experimentally.