Samples of human dental enamel and hydroxyapatite were equilibrated at 5 mg/40 ml for 9 days at 37 degreesC with acetate buffers adjusted to a range of saturations with respect to hydroxyapatite. Sigmoidal apparent solubility distributions, in which the fraction dissolved was plotted against - log(ion activity product for hydroxyapatite) (pI(HA)), were constructed. About 10% of the hydroxyapatite and 14% of the enamel was very soluble, dissolving even at pI(HA) 55. The apparent solubility distributions for both solids were invariant with pH (4.5, 5.0, 5.5), within experimental error, showing that solubility was controlled by a phase with the stoichiometry of hydroxyapatite, probably in the form of a surface layer or complex on the crystals, in agreement with other studies on carbonate-apatites and bone mineral. The pI(HA) at 50% dissolution was employed as an average value. The pI(HA) (50%) values for pooled data (58.76 for enamel and 60.17 for hydroxyapatite) were lower than the respective pI(HA) previously measured by conventional equilibration techniques. However, the average pI(HA) measured for enamel was higher than that obtained by the same technique in another study, possibly because of differences in specimen preparation and equilibration time. The possible implications of the findings for understanding the process of dental caries are discussed. (C) 2004 Elsevier Inc. All rights reserved.