A variety of sulfuric acid hydrate films, formed by the co-deposition Of SO3 and H2O on a cooled substrate, have been analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The films were produced using procedures developed in recent infrared spectroscopic studies. Spectra have been shown to consist of a series of identifiable fragments, the change in intensities of which can be related to changes in temperature and the relative abundance of H2O during the deposition of the film under UHV conditions. Depth profiling of the films shows clear evidence of separate surface species on some films and supports the existence of surface molecular hydrates over a stable bulk hydrate film.