Essential differences in electronic structure and phase composition of silicon nanowires were demonstrated by X-ray spectroscopy technique. Silicon wires massives have been produced by metal assisted wet chemical etching of the crystalline silicon substrates. The formed massives and individual nanowires have been studied by scanning and transmission electron microscopy. Electronic structure and phase composition studies of the surface and near surface layers of the massives were performed by laboratory based Ultrasoft X-ray Emission Spectroscopy and synchrotron based X-ray Absorption Near Edge Structure Spectroscopy. It is shown that the sample that are morphologically more developed and formed on a substrate with low resistivity is considerably more strongly subjected to oxidation with noticeable formation of intermediate silicon oxides phases. The massive of nanowires formed on a substrate with high resistivity also naturally oxidized, but to a lesser extent and contained the phase of crystalline silicon with the increase of the probation depth. The anomalous effect of the synchrotron radiation interaction with the silicon wires structure formed at the low doped substrate is detected and explained.