In a thermal vacuum test chamber total hemispherical emissivities for different materials have been determined. Attention was focussed on coatings which have a clear relevance to application on space craft. Conditions during the measurements resembled closely the environment in space. Materials investigated include two black paints, electropolished steel, polished gold and aluminum tape. For gold and aluminum the drastic increase in emissivity as a consequence of contamination of the surfaces by cryodeposited water was quantitatively confirmed. Proton irradiation of black paint did not cause a significant change in emissivity.