Calcium titanate crystallization has been observed following the hydrothermal reaction of titanium dioxide (TiO 2 ) and calcium in alkaline solution at 250 °C, using time-resolved in situ energy-dispersive diffraction. The nature of the calcium source used was found to influence the type of calcium titanate formed; CaTiO 3 was produced when calcium carbonate was used and Ca(TiO 2 ) 2 (OH) 2 was produced when the calcium aluminosilicate, Ca 3 Al 2 (SiO 4 )(OH) 8 was used. Temperature had a significant effect on the formation of CaTiO 3 , as observed in the range 200-250 °C. A less-significant effect was apparent for the formation of Ca(TiO 2 ) 2 (OH) 2 in the same range. Kinetic analysis of the crystallization of both calcium titanates was performed in the region 200-250 °C by applying the Avrami-Erofe'ev equation to the growth data. Activation energies of 89 and 58 kJ mol -1 were returned for the formation of CaTiO 3 and Ca(TiO 2 ) 2 (OH) 2 , respectively.