The existence of atomic-silicon cryptates in siloxanic networks has been studied theoretically via high level quantum mechanical calculations. Modeling with model molecules the candidate sites to host atomic silicon, we found that metastable adducts can be formed only in regions where the siloxanic network is not subjected to steric constraints; stationary states are instead impossible in highly reticulated siloxanic networks. This analysis suggests that the atomic silicon injected into the oxide during thermal oxidation of silicon with O-2 may be trapped as a metastable adduct at the oxide surface. (C) 2004 Elsevier B.V. All rights reserved.