Sequestration of calcium phosphate by caseins occurs in the Golgi region of mammary secretory cells during lactation, where it helps to prevent calcification of the gland and to deliver high concentrations of calcium and phosphate to the neonate in the form of milk. Calcium phosphate nanoclusters are formed when a core of amorphous calcium phosphate is sequestered within a shell of casein or casein phosphopeptides. The nanoclusters can form spontaneously from a supersaturated solution or by dispersion of a precipitate of calcium phosphate, demonstrating that they are thermodynamically stable complexes. The average size and chemical composition of the complexes are largely independent of the solution conditions (pH, temperature, peptide concentration, salt composition and rate of reaction) under which they form. Larger, metastable, colloidal particles can form if there is not enough of the phosphopeptide to sequester all the calcium phosphate, or, transiently, if the salt and peptide solutions are mixed together without sufficient care. A thermodynamic model of the sequestration process is presented which makes use of an invariant ion activity product observed in nanocluster-containing solutions. In any given solution that has thermodynamic stability, the extent of the sequestration reaction can be calculated from the empirical formula of the nanoclusters using the criterion that the solution should have the equilibrium value of the invariant ion activity product. Other members of the paralogous group of secretory calcium-binding phosphoproteins to which caseins belong may also be able to sequester calcium phosphate in biological fluids such as saliva and in the extracellular matrix of mineralizing tissues.