The construction of a model of the calcium orthophosphate solubilities has been described in detail. The model gives a true and easily comprehensible visualization of the complex interrelationships existing in phosphate systems. Examples of its application have been given as regards the stability fields, the biologic metastability, the congruent solubility, and the kinetics of dissolution and precipitation processes. As an instrument of assessing the solubilities in practical work, the usefulness of the model is further increased by including diagrammatic representations of the solubilities of Fap, CaF2and CaCO3. The model virtually covers the whole field usually encountered in solubility studies. It is based upon an ionic strength of 0.05, and any complexation, ionic interaction, or specific ionic effects have been disregarded It is possible, however, to make the model to any scale and to construct it with a view to any purpose required.