Sulphuric acid hydrate films have been formed under UHV conditions by the co-deposition of SO3 and H2O on a cooled substrate and analysed using secondary ion mass spectrometry (SIMS). The deposition conditions follow procedures reported in recent infra-red spectroscopic studies. Spectra contain characteristic fragments, the change in intensities of which can be related to changes in temperature and relative abundance of H2O during deposition of the film under UHV conditions. The reaction of such films with varying amounts of ammonia has been monitored and changes in relative peak intensity have been explained using ammonium sulphate and bisulphate reference spectra. (C) 2004 Elsevier B.V. All rights reserved.