The overall objectives of the author's research are to increase the understanding of the mechanism of caries inhibition by the interaction of fluorides with teeth through systematic studies of the chemistry of the products formed when fluorine containing materials react with tooth material (hydroxyapatite). Since dental enamel consists primarily of hydroxyapatite (with numerous impurities) any theoretical models designed to explain the mechanism of tooth decay and/or its inhibition by fluoride must be based on reliable values for the thermodynamic properties of both hydroxyapatite and fluorapatite. Necessary data for hydroxyapatite are available in the literature. However, much of the published data on fluorapatite suffer from lack of pure samples or failure to recognize the true conditions of equilibrium between fluorapatite and water, i.e. equilibrium is attained not between pure fluorapatite and water but between solid solutions of fluorhydroxyapatite, Ca5(PO4)3Fx(OH)(1-x), and water. It is the specific objective of the research outlined in this proposal to measure the aqueous solubility product of fluorapatite using fluorhydroxyapatite samples and the recently published theory of solid solution solubility developed by the author. Care will be taken in the preparation of reliable, homogeneous, samples and to assure the attainment of true equilibrium. The value of the solubility product will be measured as a function of temperature to provide values for deltaH and deltaS of ionization.