The mechanism by which calcium-deficient hydroxyapatite can become carbonate apatite has been investigated by means of a monofluorophosphate competition experiment, measurement of phosphate exchange and uptake of 14C-labelled bicarbonate in the presence of various concentrations of orthophosphate. It was found that acid phosphate groups were displaced from the crystal surface by treatment with bicarbonate. This did not result in a greater acid solubility of the mineral. Moreover, removal or replacement of the acid phosphate groups from the crystals prevented them from acting as nucleators or centres of crystal growth in a calcifying medium. Therefore, when dental mineral is formed in the presence of bicarbonate, some of the acid phosphate groups are displaced, limiting the growth of the crystals. This results in a proportion of small crystals being present having a high CO2 content and being readily dissolved during carious or acid attack. This accounts for the observed inhibition of calcification by bicarbonate and for the preferential loss of CO2 during caries.