Structure and spectra of amorphous HCl monohydrates are examined using on-the-fly MD and FTIR spectroscopy. Although the crystal monohydrate is composed of H3O+ and Cl- ions, the local structure of amorphous HCl:H2O similar to 1:1 hydrate depends strongly on preparation conditions. Ultra-cold deposits obtained in presence of excess HCl are largely molecular, while warming produces an amorphous solid of primarily H3O+ and Cl- ions with a fraction of un-ionized molecules. By contrast, 1:1 deposits experience a slight deficiency of HCl from segregation of HCl as HCl (s). The result is a distinctly different spectrum indicative of enrichment in Zundel ions (H5O2+). The simulations permit identification of composition-dependent FTIR characteristics with unique local structures. (c) 2006 Elsevier B.V. All rights reserved.