Unsupported Fe-Mn-K catalysts prepared earlier by incorporating potassium in-situ during coprecipitation of iron- and manganese oxides have been found to exhibit good catalytic activity and high alkene selectivity in the C2-C4 range, during syngas conversion. The effect of manganese on the alkali promoted iron catalysts towards hydrocarbon synthesis was studied here. Manganese in moderate amount promotes catalytic activity as well as selectivity of lower alkenes. Using a catalyst containing about 20 mol-% Mn and 0.5% K, at 673 K, 3 kg/cm2 pressure, WHSV = 800 h-1 and H-2/CO = 1, it was possible to obtain good carbon monoxide coversion with 70-80% lower alkenes without formation of excess methane and carbon dioxide.