CO hydrogenation over a catalyst with the nominal composition of 6.3% Mn-2.5% Ru/Al2O3 was studied in a now system with a differential fixed-bed reactor operating under atmospheric pressure. The temperature was varied between 553-613 K, H-2/CO ratio 1-3 and space velocity 5100-27600 cm(3)/(g h). The presence of manganese caused the product distribution to shift to lower olefins. For this catalyst system, secondary alkene hydrogenation was found to be significant. The activity and selectivity pattern of the catalyst altered appreciably at/below and above 573 K. It is hypothesized that at/below 573 K, monolayer carbon deposits and above 573 K multilayer carbon deposits affect the performance of the catalyst.