Homogeneous (unseeded) nucleation and precipitation kinetics of Ca and Mg phosphates from modified seawater solutions with variable pH, Mg2+, HCO3−, F−, and organic acid concentrations were investigated at 20°C. The induction period of nucleation and the composition of precipitated solid phases were determined as a function of saturation state (6–2000 with respect to octacalcium phosphate (OCP)), Mg2+/Ca2+ activity ratio (0–10) and solution pH (7.2–9.1). The addition of Mg2+, HCO3−, F−, polycarboxylic, aromatic, and amino acids along with variations in pH has a weak effect on the nucleation kinetics of Ca–Mg phosphates in seawater solutions with a 35% salinity. Mg-bearing solutions produce an amorphous precipitate with a Ca : Mg : P molar ratio varying from 6.5 : 1 : 5 to 2.6 : 1 : 2.5 depending on (aMg2+/aCa2+) ratio. The precipitation of this amorphous phase in Mg-bearing solutions corresponds to the formation of a metastable hydrous Ca–Mg phosphate ((Ca,Mg)4H(PO4)3·xH2O) as a precursor of apatite which normally precipitates in Mg-free solutions. Spontaneous inorganic precipitation of Ca phosphate in most natural aquatic systems is kinetically impossible.