Hydridosilsesquioxane-based model systems have been developed over the past five years to help elucidate the structure and reactivity properties of silicon/silicon oxide interfaces. In this paper, the assignment of infrared bands as Si-O or Si-H derived is explored via deuterium labeling studies. In particular, the issue of the presence of adventitious water in the model interfaces is directly addressed via the chemisorption of deuterated- spherosiloxane clusters (D8Si8O12) onto clean Si(100). The experiments definitively demonstrate that there are no water-derived features detectable by RAIRS in the model interfaces. This rules out water contamination as a possible explanation for the 1.0 eV shifted feature observed in Si 2p core- level spectroscopy for the model systems. This work is further supported by cluster and water co-dosing experiments on Si(100), which address the spectroscopic sensitivity of surface silicon-hydrides in this system. Finally, the first surface crystallographic studies of the model interfaces performed with LEED are presented. The chemisorbed hydridosilsesquioxane model systems are shown to retain the 2 x 1 reconstruction of the original Si(100) surface.