The action of sodium monofluorophosphate on Syntheti chydroxyapatites has been studied using pure mono- syntheticfluorophosphate solutions.Monofluorophosphate-treated hydroxyapatite was less soluble in acid buffer than fluoride-treated mineral of similar fluorine content. During the reaction some orthophosphate was liberated from the solid and addition of orthophosphate to the monofluorophosphate inhibited subsequent uptake of fluorine by the hydroxyapatite. The FPO3 anion was present in the treated hydroxyapatite and was shown to have replaced the hydrogen phosphate group of the calcium-deficient hydroxyapatites. In this way the capacity of a calcium-deficient hydroxyapatite for uptake of monofluorophosphate was related to its HPO4 content; i.e. its Ca:P molar ratio. Replacement of the acid phosphate groups of the hydroxyapatite, either by FPO3 substitution or by thermal condensation to pyrophosphate reduced the isotopic exchange of orthophosphate by the mineral.