The results of our theoretical and experimental study of electronic structure of poly(sulfur nitride) are presented. The S L2,3 (3s→2p transition) and N Kα (2p→1s transition) X-ray emission spectra (XES) of (SN)x single crystal are measured using synchrotron radiation. The experimental spectra are compared with the results of ab-initio TB-LMTO band structure calculations of 3D solid (SN)x and molecular (S2N2)2 crystals. It is found that the molecular (S2N2)2 is an insulator with a band gap about 2.7 eV whereas the solid (SN)x is metallic. According to the theoretical and experimental information, the S 3p states dominate the density of states (DOS) at the Fermi level and provide the metallic and superconducting properties of this compound.