The Mg(HCO3)2 solution obtained by pressure carbonation of hydrated MgO is decomposed by heating to form MgCO3 of high purity. Factors which influence the precipitation of MgCO3, such as temperature, concentration, seeding, aeration and stirring, have been studied. Results of tests carried out with A.R. grade MgCl2 and Na2CO3 at various temperatures are reported. The activation energies calculated from the rate equations were 63-85 kJ mol-1, which suggests a chemically controlled reaction mechanism.