A possible chemical process occurring during caries reversal is conversion of acidic calcium phosphates to apatite. The role of fluoride in this process is of particular interest. The effects of fluoride on the rate of CaHPO4 hydrolysis at 37.4 degrees C were studied by a multimethod analysis involving X-ray diffraction, analyses of variations in solution chemistry, and observation of microstructural evolution. Hydrolysis in low NaF concentrations results in the formation of discrete fluorapatite crystals on the surfaces of the CaHPO4 crystallites. At CaHPO4/NaF molar ratios from approximately 9:1 to 10:2, fluorapatite formed in approximately 3 h as the only crystalline product and complete hydrolysis of CaHPO4 occurred; a pH value as low as 2.4 was attained with solution species being predominantly sodium phosphate. At NaF concentrations beyond those which result in pH minima, fluorapatite and CaF2 are the crystalline products. At 0.6 M NaF, pseudomorphs composed of fluorapatite and CaF2 crystals form without developing morphologies characteristic of individual fluorapatite and CaF2 crystals. CaHPO2 can hydrolyze completely to fluorapatite and CaF2 within a few hours depending on NaF concentration and liquid-to-solids ratio.