The x-ray photoelectron spectroscopy (XPS) technique has been used to investigate the composition of semi-insulating polycrystalline silicon (SIPOS) films having oxygen contents of 10 and 35 at. % prepared by low pressure chemical vapor deposition. XPS analysis has demonstrated that the him compositions can be qualitatively described by means of the five Si-SixO4-x tetrahedra (with 0 less than or equal to x less than or equal to 4 and integer) predicted by the statistical random bonding model (RBM). However, the quantitative analysis of the XPS spectra has demonstrated that the concentrations of the various tetrahedra found in the SIPOS films are remarkably different from those predicted by a statistical approach, i.e., by assuming that each Si atom forms with equal probability bonds with either Si or O. We have also found that the composition of high temperature (up to 1000 degrees C) annealed films further departs from that predicted by the RBM model; indeed, the anneal promotes the decomposition of partially oxidized Si-SixO4-x tetrahedra in Si-Si-4 tetrahedra (that form Si nanocrystals) and Si-O-4 tetrahedra (that enrich the oxygen content of the amorphous phase).