The transformations and the state of cobalt species during steam reforming of ethanol over Co/CeO(2) were investigated using in situ X-ray diffraction, controlled-atmosphere X-ray absorption fine structure, and Xray photoelectron spectroscopy as well as steady state activity measurements. The catalyst was pre-treated under an oxidizing or reducing atmosphere prior to characterization and activity testing to yield a Co(3)O(4)-rich or a Co(0)-rich surface, respectively. While CO(3)O(4) was found to be inactive for ethanol steam reforming, gradual activation of the oxidation-pretreated catalyst with temperature through reduction in Co(3)O(4) took place under reaction conditions, and, over the activated catalyst, a mixture of both CoO and metallic Co were observed. Over the reduction-pretreated catalyst, metallic Co was partially oxidized to CoO during steam reforming of ethanol. The extent of cobalt reduction was observed to be independent of the initial state of the metal on the catalyst surface, and cobalt phase had the same composition under reaction above 450 degrees C. (C) 2011 Elsevier Inc. All rights reserved.