The aerial oxidation of aqueous suspensions of ferrous hydroxide precipitated from ferrous oxalate and caustic soda can lead to an iron (II)-iron (III) hydroxy-oxalate of the pyroaurite group, a GR(CzO42) Green Rust. As other GR compounds, it is unstable with respect to the action of oxygen and oxidises later on. Its chemical composition was determined to be [Fe~ 6 Fern2 (OH)1612+[C2042" • nH20], with n more likely equal to 3 on the basis of structural considerations. The composition does not vary and the Fe (II) / Fe (III) ratio in the compound is measured by means of transmission MOssbauer spectroscopy at 78 K and 20 K hi the range from 2.8 to 3.2 for various samples at various stages of the reaction. GR(C2042) is paramagnetic at both temperatures and is unambiguously distinguished from ferrous hydroxide, the initial reactant, and magnetite, the main f'mal product, which are magnetically ordered at 20 K. The spectrum of the GR compound is composed of three quadrupole doublets, one due to the Fe(IIl) cations characterised by a small quadrupole splitting AE o of 0.40 mm s", and two due to the Fe(]I) cations, characterised by larger AE o values of about 2.55 and 2.85 mm s ~. Finally, from the observed equilibrium conditions between ferrous hydroxide and GR(C2042"), the standard free enthalpy of formation of GR(C2042-) was computed to be : AG°f {[Fe~6 Fern 2 (OH)~] 2+ [C204; • 3H20]} = -5383 4-3 kJ mol ~.