Theoretical absorption spectra of HCl trihydrate, HCl hexahydrate, and HNO3 trihydrate in the mid-IR have been calculated using the SIESTA suite of programs, a DFT method especially designed for periodic systems of large size. Infrared intensities are obtained from the macroscopic polarization changes per normal mode. The theoretical results can account globally for the observed experimental features. In all spectra, the largest absorption intensities are due to stretching vibrations of hydrated oxonium ions. Oxonium. stretching absorptions appear as broad bands in the experimental spectra, with band widths growing with increasing number of water molecules coordinated to the H3O+ ion. (c) 2006 Elsevier B.V. All rights reserved.