  We report on our first results from a mid-infrared spectroscopic study of ISM
features in a sample of deeply obscured ULIRG nuclei using the InfraRed
Spectrograph (IRS) on the Spitzer Space Telescope. The spectra are extremely
rich and complex, revealing absorption features of both amorphous and
crystalline silicates, aliphatic hydrocarbons, water ice and gas phase bands of
hot CO and warm C_2H_2, HCN and CO_2. PAH emission bands were found to be
generally weak and in some cases absent. The features are probing a dense and
warm environment in which crystalline silicates and water ice are able to
survive but volatile ices, commonly detected in Galactic dense molecular
clouds, cannot. If powered largely by star formation, the stellar density and
conditions of the gas and dust have to be extreme not to give rise to the
commonly detected emission features associated with starburst.
