Homogeneous (unseeded) precipitation of calcium and magnesium carbonates in the CaO–MgO–CO2–H2O system in supersaturated solutions was investigated. The induction period (τ) of spontaneous nucleation and the composition of precipitated solid phases were determined. The influence of saturation degree (chemical affinity) and Mg2+/Ca2+ activity ratio in solution on the kinetics of CaCO3 nucleation was studied. The order of CaCO3 homogeneous nucleation equals to 4. The surface energy of CaCO3 nucleus was measured to be 63 mJ/m2. The main factor which controls the Mg content in the solid phase being formed during unseeded precipitation of carbonates is the saturation index of MgCO3 in solution.