Ultrafine Mo-Co-K catalysts were prepared and tested for higher alcohol synthesis. The catalysts exhibited high catalytic activity. The effect of the mole ratio of cobalt and molybdenum in the catalysts upon the catalytic performance of higher alcohol synthesis was investigated. Among the ultrafine Mo-Co-K catalysts, the best one corresponded to the Co/Mo mole ratio of 1 :7. The XPS spectra revealed that molybdenum was present in two species: Mo6+ and Mo4+ on the surface of reduced catalysts, and the Mo4+ species consent depended strongly on the Co/Mo mole ratio. The selectivity towards higher alcohols was found to be related to the Mo4+ species content. A linear relation between the selectivity and Mo4+ species content led to the conclusion that the Mo4+ species was the main active species for higher alcohol synthesis over the ultrafine Mo-Co-K catalysts.