Solubility isothe nns of hydroxyapatite, Ca5(PO.h OH (OHAp) , pre pa red by titrating a boiling aqueous suspe nsion of Ca(O HJ, with 0 .5 M H3PO., we re detennined in the te rnary system Ca(OHJ,-H3PO.-H20 at 5, 15, 25, a nd 3 7°C in the pH ra nge 3 .7-6. 7 by equilibration with dilute H3PO. solutio ns. The solubility product K" dete rmi ned as a fun ct io n of te mpe rature by a gene ra lized least-squa res procedure from 4 1 ex pe rimenta l points, is g ive n by the equationThe values of K s and it s dis persion at 25 a nd 37°C a re 3.04 (0.25) a nd 2.35 (0.27) X 10-59 . The re is a max imum in K s near 16°C, whic h may be d ue to the fo rm of te mperature de pendence found earlier fo r the sta bilit y constants of the ion pa irs Ca H2POt a nd Ca HPO~.The re lati ve pos itio ns of the isothe rms s how that OHA p has a negati ve the rma l coeffi cient of solub ility. T hermod ynam ic fun ctions for the di sso lutio n of the sa lt a re report ed .T he so lubility data previous ly re pOl1ed by others fo r OHAp at 25°C were rev iewed . The solubility prod uc ts obta ined by three of these in vestigators were compara ble with our value of 3 .0 X 10-59 ; thei r data were reeva luated by the method desc ri bed he re. We conclude that the best value fo r the so lubility p rod uct at 25 °c is 4.7 (2 .0) X 10-59 . Key wo rd s: Dissolution; hydroxyapatite; ion pa irs; sol ubility; so lu b ilit y isothe rms; so lu bility prod uct; the rmal coeffi c ient of solubilit y; thermod ynamics; tooth mine ra l.