Potassium-promoted cobalt molybdenum sulfide (K-CoMoSx) mixed alcohol synthesis catalysts were operated from 118 to 3969 h for the purpose of studying catalyst deactivation. Continuous and discontinuous sulfiding with H2S and methyl sulfides was considered. Fresh and discharged catalysts were analyzed via X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). When continuously sulfided, selectivity was maintained for thousands of hours. Without sulfur cofeed, catalysts exhibited a change in selectivity from alcohols to hydrocarbons over periods of hundreds of hours. Sulfur deprivation resulted in oxidization, carburization, and coking of the catalyst surface and segregation of cobalt into crystalline C0(9)S(8). It is suggested that in the absence of a sulfiding agent, the catalyst surface becomes more acidic (oxidized) promoting dehydration of alcohols (selectivity change) and coking (blocking active sites). Reintroduttion of H2S may reverse oxidation on non-coked surfaces. Proper sulfur maintenance may render catalysts operable for years without need of regeneration or replacement. (C) 2013 Published by Elsevier Inc.