Thin films of SiO deposited on Si substrates by thermal evaporation have been characterised by the analysis of infrared absorption bands and optical constants determination. The structure of the films has been studied in terms of the RBM (Random Bonding Model) and a deviation from the pure RBM structure has been found in the relative concentration of the different tetrahedra Si-O-y-Si4-y (y = 1-4) and in the presence of an additional contribution Of Si-O-4 tetrahedra confirmed by the optical properties, Comparative study of films deposited under different conditions and with different thickness suggests that the local arrangement of Si and O atoms, the oxygen content of the films and morphological changes in the films must be considered to account for all the effects observed.