We report a Si K-edge X-ray absorption fine structures (XAFS) study of silicon monoxide (SiO), the starting material for silicon nanowire preparation, its silicon nanowires, and the residue after the preparation of the starting material. The silicon nanowires were condensed onto three different substrates, (i) the wall of the furnace quartz tube, (ii) a porous silicon substrate, and (iii) a Si(100) silicon wafer. It was found that the Si K-edge XAFS of SiO exhibits identifiable spectral features characteristic of Si in 0 and 4 oxidation states as well as in intermediate oxidation states, while the SiO residue primarily shows features of Si(0) and Si(4). The XAFS suggest that SiO is not exactly a simple mixture of Si and SiO2. The silicon nanowires produced by the process exhibit morphology and luminescence property variations that depend on the nature of the substrate. X-ray excited optical luminescence (XEOL) at the O K-edge suggests an efficient energy transfer to the optical decay channel. The results and their implications are discussed.Key words: silicon nanowires, thermal evaporation, silicon monoxide, X-ray absorption fine structures, X-ray excited optical luminescence.