The oxidation of three bituminous coals, ranging from low rank (84.3 wt % carbon) to high rank (88.3 wt % carbon) in the presence and absence of water vapor, has been studied using X-ray photoelectron spectroscopy (XPS) and gravimetric techniques. The effects of wetting the coal surface with liquid water, following oxidation with humid air, have also been studied. On oxidation, the sample weight increased as time increased up to at least 40 h. XPS measurements show that, when the coal was oxidized in the presence of dry oxygen, only minor amounts of the C=O groups were formed; however, the amount of these and the C-O groups increased when the coal surface was wetted. The water-vapor adsorption properties of unoxidized coals and both types of oxidized coals were also studied. Oxidation with dry oxygen caused a decrease in water-vapor uptake, compared to that observed for an unoxidized coal. However, if the surface was wetted, the surface water catalyzed both oxygen and water uptake. Thus, two types of water have been recognized, and only that which is delivered by surface wetting is capable of catalysis. Catalytic water is probably chemisorbed and other water is physically adsorbed.