Hydrated sulphuric acid in dense molecular clouds

We consider sulphur depletion in dense molecular clouds, and suggest hydrated sulphuric acid, H(2) SO(4) . n H(2) O, as a component of interstellar dust in icy mantles. We discuss the formation of hydrated sulphuric acid in collapsing clouds and its instability in heated regions in terms of the existing hot core models and observations. We also show that some features of the infrared spectrum of hydrated sulphuric acid have correspondence in the observed spectra of young stellar objects.