X-ray diffraction technique and hard mode infrared spectroscopy were used to study the thermal behaviour of synthetic titanite, CaTiSiO5, two natural titanite samples and iso-structural malayaite, CaSnSiO5. Both pure minerals show a thermal anomaly near 500 K. Specific heat measurements show an intermediate phase in synthetic titanite. Impurities in titanite tend to suppress the discontinuity near 500 K. The β-γ transition near 820 K is not affected by Fe and Al defects in the natural material. Infrared studies in malayaite show a change of the temperature dependence of the phonon frequencies. X-ray experiments show weak structural rearrangements with heating but no evidence of a symmetry breaking phase transition near 500 K. The character of the 500 K anomaly in malayaite is more similar to the β-γ transition (825 K) than the α-β transition (496 K) in synthetic titanite.