We present the results of a comprehensive series of measurements on glow‐discharge (plasma) ‐deposited silicon nitride films SiNx:H, withxin the range 0<x<1.2. Optical spectroscopy in the visible and infrared regions is used to investigate the nature of the bonding and to assess the role of hydrogen. With increasingx, in the rangex<0.7, an increase in the concentration of Si‐H bonds results in an increase in the total hydrogen content; at higherxthe rise in the N‐H concentration produces a small increase in the hydrogen content, but even for these samples most of the hydrogen is bonded to silicon. The optical absorption edge due to band‐gap transitions broadens with increasingxdue to a change in the nature of the valence band from Si‐Si bonds to N lone‐pair states. Electrical conductivity at high fields and magnetic resonance measurements give information about the defects in the band gap. These results support the Robertson–Powell model in which the principal defect in the band gap of silicon nitride is the silicon dangling bond.