Summary: The infra-red spectrum of untreated thaumasite Ca3H2(CO3/SO4)SiO4.13H2O, as well as the spectra and X-ray diffraction patterns of thaumasite heated in air at 200, 360, 550, 725, 800, 900, and 1150° C were investigated. The release of CO2 and H2O upon heating was followed. It was established that thaumasite changes between 200 and 550° C to a glass-like, amorphous state. Ca2SiO4-polymorphs and anhydrite are formed upon heating at temperatures up to 800° C. From 900° C calcium silicosulphate and anhydrite are the thermal decomposition produets. These results are used in the assignment of the IR-absorption bands under 1400 cm−1 of untreated thaumasite, which are shown to be of a composite nature, i.e. attributable to overlapping vibrations of both sulphate and silicate tetrahedra.