A cobalt/ceria catalyst, promoted by ruthenium, has been found to be selective for production of lower alkenes and higher n-alkanes from synthesis gas, showing minimal loss of activity with time (>300 h) and low water-gas shift activity. Mixing the catalyst with an equal weight of either a mildly acidic binder (bentonite) or a basic one (magnesium oxide) resulted in little change in activity and carbon selectivity, including overall alkene yields. However, use of bentonite led to substantial double bond shift in alkenes at C4.5, with resultant reduction of 1-alkene content. No such isomerisation occurred in the case of magnesium oxide; the hydrocarbon products were distributed almost equally between gas and liquid phases under most conditions. Detailed gas phase analysis revealed mainly 1-alkenes in the range C26. The liquid phase, by contrast, consisted mainly of n-alkanes, extending to C28. The fuel quality of the liquid fraction has been assessed.