A model is presented for the pathway of reactions of the mineral of tooth enamel during caries. It is based on a mathematical simulation of the caries process (van Dijk, Borggreven and Driessens, 1979) and knowledge about the variable solubility behaviour of calcium phosphates (Driessens, van Dijk and Borggreven, 1978). According to this model the surface of the crystals in the intact superficial layer of enamel will transform into a relatively pure fluor-hydroxyapatite; in the lesion a mineral with a brushite- or monetite-like composition will be formed, while at the bottom of the lesion an increasing amount of the original carbonatoapatite will be found. The experimental studies of Arends and Davidson (1975) and of Featherstone, Duncan and Cutress (1978) corroborate these results. The model also explains why demineralization take place at some distance under the enamel surface, and predicts that the intact superficial layer of enamel over a lesion becomes thicker with increasing pH of the plaque fluid and with its degree of saturation with respect to fluor- and hydroxyapatite (Larson, 1974 b). Sobel's finding (1960) that high-carbonate teeth are more susceptible to caries than low-carbonate teeth is consistent with the present model.