A combined theoretical and experimental study of crystalline hydrogen chloride hexahydrate HCI - 6(H2O) has been performed. This species is stable in the range of atmospheric pressure and temperature, and some apparent discrepancies existed between previous crystallographic and vibrational studies. This work presents theoretical calculations based on the SIESTA method that allow a refinement of the geometrical structure and a prediction of the vibrational modes of the crystal. The experimental part consists in new reflection-absorption infrared spectra, that are analysed and assigned with the help of the theoretical results, and compared to previous assignments of infrared transmission spectra. This research supports the equivalence of the samples prepared by different methods in previous experimental investigations. (C) 2004 Elsevier B.V. All rights reserved.