AbstractA reconsideration of literature data reveals that the incorporation of vacancies, Na, K, Sr, Cl, and CO3 into hydroxyapatite increases its apparent solubility product, whereas that of F decreases it. It is further shown that the composition of saliva is regulated so that a steady state is reached for erupted teeth whereby equilibrium between mineral and saliva is close to the two‐phase boundary of apatite and dicalciumphosphatedihydrate (DCPD). The theory explains qualitatively why and how gradients occur of F, Na, and CO3 in surface enamel, why the surface layer remains nearly intact during caries and why calcifications occur in the arteries and not in the veins. – It is assumed that the incorporation of Mg and citrate ions can increase the apparent solubility product of tricalciumphosphate (TCP) and octacalciumphosphate (OCP) respectively. On this basis an estimate is made of the relative position of the stability fields of apatite, DCPD, TCP, and OCP. This estimate indicates why these calcium phosphates may occur simultaneously in dental calculus, bone, dentine and pathological calcifications. If both the effect of crystal size and that of incorporation of foreign ions are taken into consideration, it can be presumed that calcified tissues are in momentary physico‐chemical equilibrium with the body fluids bathing them.