The oxygen dissolution in metallic zirconium has been investigated by electro-chemical and X-ray techniques. The mechanism of the zirconium oxidation process is proposed. It is shown that during the first stage of the process, oxygen dissolves in the metal matrix acting as an electron donor. The region of oxygen solid solutions in zirconium reaches 25 at.% oxygen. After oxygen saturation of the metal surf ace the second stage starts; zirconium oxides form.