Fourier transform infrared spectroscopy and synchrotron X-ray powder diffraction methods are described for studying cement hydration chemistry at temperatures up to 200 deg C, covering the normal temperature range of wellbore cementing. The methods provide complementary information on the transformation of silicate, ferrite and sulphate minerals. The thermal decomposition of the cement mineral ettringite is shown to occur at 114 deg C in a sealed system in contact with water. The FTIR spectrum of a well cement slurry hydrating at 150 deg C and 2000 psi is analysed. The anomalous thickening time behaviour of certain cements around 75-100 deg C is discussed in the light of new data on the hydration of a Class G cement at 65 and 95 deg C, with and without retarder.