Calcium phosphates (CaP), the sparingly soluble salts of phosphoric acid, are of particular importance in various interdisciplinary fields of science such as geology, biology, chemistry and (bio) medicine. They are widespread in living organisms and in nature. Calcium phosphate research is today focused on a better understanding of the processes of normal and pathological biomineralization and the development of new materials with improved properties. Although CaP deposition diagrams have been investigated experimentally in conditions close to physiological ones, their theoretical deposition diagrams have not been developed. In this paper, theoretical precipitation diagrams of four biologically important phases of calcium phosphate (amorphous calcium phosphate (ACP), octacalcium phosphate (OCP), calcium hydrogen phosphate dihydrate (DCPD) and hydroxyapatite (HA)) are made in systems without additives and with the addition of magnesium, citrate (Cit) or both additives (t = 25 °C and pH 7.4): CaCl2 – Na2HPO4, CaCl2 – Na2HPO4 – MgCl2 (c = 0.0015 mol dm^–3), CaCl2 – Na2HPO4 – Na3Cit (c = 0.000125 mol dm^–3), CaCl2 – Na2HPO4 – MgCl2 (c = 0.0015 mol dm^–3) – Na3Cit (c = 0.000125 mol dm^–3). The obtained results showed that, in all precipitation systems, depending on the composition of the precipitate, four precipitation areas can be distinguished, in accordance with the solubility of each salt. The differences in the shape of the theoretical precipitation boundary for DCPD with respect to the other CaPs studied indicate a possible difference in the dominant processes by which dissolved ionic and other species enter the solid phase. The differences in the position of theoretical precipitation limits in different deposition systems for each phase of calcium phosphates are consistent with the experimentally observed effects that Mg2+ and Cit3- ions have on the processes of their formation.