It was found that copper surfaces, which had been exposed to a clean room atmosphere, were covered by a layer, whose chemical composition can be described by Cu(OH)2·CuCO3. This layer can effectively be removed by either a short thermal treatment in vacuum at 350°C, a hydrogen plasma treatment, or a combination of both. Ex-situ photoelectron spectroscopy measurements show little difference of the chemical composition of the surface after the respective treatments. The thermal treatment, however, gives rise to re-crystallisation of the copper film due to the difference in temperature of deposition and the anneal. Ex-situ ellipsometry measurements indicate that the hydrogen plasma not only removes Cu(OH)2·CuCO3 but also passivates the copper surface.