The calcium catalyzed steam gasification of coal has been studied by a combination of a chemical microreactor working under technical conditions and surface analytical characterization (Auger and photoelectron spectroscopy, secondary ion and thermal desorption mass spectrometry) under ultra high vacuum conditions. The reaction model proposed by McKee was confirmed, and reaction products like calcium oxide, hydroxide and carbonate were detected after exposure to gasification conditions. The catalyst dispersion was studied as a function of reaction temperature and carbon conversion. The lowering of the CaCO3decomposition temperature and the decreasing sample charging at higher temperatures indicate the good contact between the catalyst and the substrate at reaction conditions. Deactivation of the catalyst is due to two processes: the inevitable sintering of calcium to CaO and catalyst poisoning by sulfur by forming calcium sulfide.