Homogeneous thin layers of lepidocrocite, green rust incorporating chloride anions (GRCl-GRl) and green rust incorporating sulphate anions (GRSO(4)-GR2) were successfully electrodeposited onto tin dioxide substrate (SnO2). The UV-vis spectroscopy absorbance, measured just after synthesis, increases linearly with the amount of iron compounds confirming that the electrodeposition is a homogeneous process. From the spectra, the absorption coefficients were determined for the three compounds. The monitoring by UV-vis spectroscopy measurements in real time of the oxidation in the air of the two unstable compounds, GRCl and GRSO(4), shows that the two green rusts have a different behaviour due to their own structure. The presence of an isobestic point in the case of GRSO(4) translates one equilibrium with its oxidised compound. This phenomenon is not observed for GRCl, however, the characteristic variation of the absorbance in the high wavelengths gives information on its oxidation reaction. From simple measurements, fruitful information on green rusts is obtained and this experimental system may be applied to other unstable solid compounds. (c) 2007 Elsevier Ltd. All rights reserved.