The initial stage of oxidation of amorphous hydrogenated silicon nitride (a-SiN:H) films biased in ionic water at room temperature was studied. A stoichiometric and insulating a-SiN:H film is oxidized chemically to form hydrogenated silicon oxynitride and has relatively low reactivity. A Si-rich and semi-insulating a-SiN:H film reacts electrochemically; the anodic oxidation proceeds easily to form silicon oxide on the anode, and the precipitation reaction proceeds easily to form silicate or polymerized oxide on the cathode. The anodic oxidation and the cathodic precipitation reaction mechanisms of: semi-insulating a-SiN:H are discussed briefly.