Synthesis gas with a CO/H-2 ratio suitable for production of energy carriers, e.g. hydrogen, methanol, and gasoline, can be produced by gasification of solid fuels with subsequent water-gas-shift (WGS) reaction. The separation of CO2 using conventional absorption processes leads to high efficiency losses. In contrast, a membrane reactor containing a catalyst provides the opportunity for significantly reduced efficiency losses. The present work aims at developing a chemically stable membrane for separation of carbon dioxide in gasification atmosphere. The desired membrane consists of two phases, a porous ceramic oxygen ion conductor filled with molten carbonate. Both phases need to be stable under the relevant process conditions, i.e. syngas at up to 800 degrees C, and compatible with each other and the water gas shift catalyst. The potential oxygen ion conductor materials, e.g. cerium gadolinium oxide (Ce0.8Gd0.2O2.8) and cerium samarium oxide (Ce0.8Sm0.2O2.8), which are known from literature to be compatible with alkali carbonate melts, were exposed to different gasification-relevant gas atmospheres at 600-900 degrees C and subsequently investigated regarding chemical reactions. Furthermore thermodynamic properties of alkali and alkaline earth carbonate systems were studied to identify a mixture with suitable melting behavior and high chemical stability as well as low volatility under relevant syngas conditions.