Aluminas pretreated with pure water (pseudo-hydrothermal treatment) and very dilute basic aqueous solutions were used as catalysts for CO hydrogenation. Pseudo-hydrothermal treatment results in an apparent enhancement of ethylene selectivity at elevated temperature (> 673 K). The active sites seem to be coordinatively unsaturated metal and oxygen counterions (Lewis acid and base pair), which are very probably located on the most external surface of the alumina and can be coordinatively saturated by calcination in air. The precursors of the active sites probably originate from a very thin layer of boehmite beyond XRD detecting ability. They can be generated by both pseudo-hydrothermal treatment and dilute basic aqueous solution treatment of alumina.