An ellipsometric gas sensor based on the surface Plasmon resonance (SPR) effect of -43 nm thick gold layers was investigated. To protect the gold layer from contamination and to improve the detection limits, the gold layers were top-coated with 5-6 nm thick layers of organic a-C:H or with inorganic metal oxides TiO2 or ZrO2. The additional layers increased the long-term stability, whereas the metal oxide layers protect better than a-C:H. Furthermore, the additional layers decreased the detection limits by one order of magnitude in case of a-C:H and two orders of magnitude in case of the oxides. It could be shown that the detection limits also depend on the kind of preparation (sol-gel process or physical vapour deposition) of the additional layers. (C) 2010 Elsevier B.V. All rights reserved.