In this study a mathematical model for the caries process is presented. It includes diffusion of ions, dissolution of mineral and complexation of ions. The conclusions from experiments with the model are that the relative stability of the surface layer of enamel can be explained by (1) a gradient in the solubility product of the enamel, (2) a gradient in the rate constant of the dissolution reaction or (3) a gradient in the porosity of the enamel. The outcomes of the simulations suggested that the minimum set of conditions that is necessary to obtain artificially a caries-like lesion is that the bulk solution should be relatively saturated with respect to apatite. This has been confirmed by an in vitro caries experiment.