The crystal structure of the Fe(II)-Fe(III) hydroxychloride known as Green Rust one (GR) was investigated by X-ray diffraction and confirmed to be analogous with that of iowaite. It is a rhombohedral crystal [R3m, a=0.3190(1)nm, c=2.385(6)nm] consisting of Fe(OH)2 like-hydroxide sheets which alternate regularly with interlayers composed of Cl ions and H2O molecules and follow the stacking sequence AcBiBaCjCbAk..., where A, B, C are OH layers, a, b, c Fe layers and i, j, k interlayers. By means of transmission Mossbauer spectroscopy analyses at 20K of the solid phases formed during the oxidation of Fe(OH)2 into GR(Cl), it was demonstrated that the composition of the GR compound varied continuously from FeII3FeIII(OH)8Cl\nH2O, with n probably equal to 2, to approximately FeII2.2FeIII(OH)6.4Cl\nH2O. The formation of GR(Cl) involves anin situ incorporation of the Cl ions from the solution into the interlayers of GR(Cl) and a corresponding oxidation of Fe(II) to Fe(III) without any structural changes. This mechanism, which allows the oxidation of iron at the only cost of chloride intercalation, explained why GR formation would be favoured vs other possibilities such as direct ferric oxyhydroxide or magnetite formation. To confirm this assumption, in situ transmission Mossbauerspectroscopy analyses were performed on iron coupons polarised potentiostatically in KCl solutions of pH about 9. At a potential of 0.55VSHE a ferrous hydroxide layer is formed on the metal while at 0.35VSHE the corrosion product is mainly composed of GR