The precipitation of calcium carbonate from aqueous solutions has been studied at supersaturations sufficient for spontaneous precipitation but low enough to enable highly reproducible experiments to be carried out with respect to induction periods and rates of precipitation. The threshold for the onset of calcium carbonate precipitation at pH 8.60 has been established. The induction periods and rates of precipitation are both strongly dependent on supersaturation. Treatment of the kinetics data according to classical nucleation theory yielded an estimated 68 erg cm^–2 value for the surface energy of the solid precipitating. From kinetics plots it is also suggested that the number of precipitating calcium carbonate crystals increases rapidly at the start of nucleation. At a later stage the particles stop increasing in number and subsequently grow. The apparent activation energy of 155 kJ mol^–1 calculated from experiments carried out at 25, 35 and 45 °C points to a surface-controlled mechanism for the precipitation of calcium carbonate.