The formation of crystalline components in gallstones is governed by the physical-chemical factors controlling the crystallization of minerals in aqueous systems. The elucidation of the mechanism of these reactions, especially at the low supersaturations of interest in vivo, is facilitated by the use of a constant composition method in which the reactions are studied at supersaturation levels automatically sustained, potentiometrically, during the experiments. Not only is it possible to distinguish between the various theoretical models of crystal growth, but the influence of in vivo modifiers of mineralization can be investigated at the same conditions of low supersaturation. The method is illustrated for the calcium phosphate system in which it is shown that precursor phases may be stabilized kinetically for long periods of time despite the fact that the driving forces for the formation of the thermodynamically most stable phase are appreciable. These factors will have important consequences during the precipitation of mineral phases in the formation of gallstones.