Synthetic calcium silicate hydrates were hydrothermally prepared between 1OO°C and 200°C. Tobermorite, xonotlite, hillebrandite, gyrolite, kilchoanite, calciochondrodite, ‘P-phase’, α-dicalcium silicate hydrate and tricalcium silicate hydrate were identified by X-ray diffraction techniques and scanning electron microscopy. The products were investigated by infra-red spectroscopy and the results compared with those obtained in the previous literature. In general, the spectra are an improvement on the previous data and, in some cases, extend the spectra to lower wave numbers. Impurities which were not identified by X-ray diffraction have been identified in the infra-red spectra. Infra-red spectroscopy, in association with other techniques, is a useful aid in phase characterisation. Class G and class J cement spectra are complicated by chemical impurities. Investigation by infra-red indicates that, while impurities can produce slight wave number shifts and alter the relative intensity of peaks, phases may still be identified by their characteristic wavebands.