An investigation has been carried out of the effects of Mn promotion on the activity and product selectivity of Co/SiO2 for Fischer-Tropsch Synthesis (FTS). All experiments were conducted with an H-2/CO feed ratio of 2.0 at either 1 atm or 10 atm and a temperature of 493 K. At 1 atm, the rate of CO consumption decreased with CO conversion, whereas the selectivities to all products were unaffected. However, the olefin to paraffin (O/P) ratio of the C-2-C-4 fraction decreased with increasing CO conversion due to increased hydrogenation of olefins with increasing space time. Mn promotion increased the rate of CO consumption at Mn/Co ratios of 0.05, decreased the selectivity to methane, and increased the selectivity to C5+ products, but had no effect on the selectivity to C-2-C-4 products. Raising the pressure to 10 atm increased the CO consumption rates for both unpromoted and Mn-promoted Co/SiO2, but the rate of reaction was lower for the Mn-promoted catalyst due to a higher level of CO inhibition. CO conversion at 10 atm had no effect on the rate of CO consumption for Co/SiO2 and caused a slight increase in the rate for Mn-promoted Co/SiO2. The intrinsic O/P ratio was higher at 10 atm than at 1 atm. In contrast to what was observed at 1 atm, at 10 atm, C-2-C-4 of the C-2-C-4 fraction incorporated into growing hydrocarbon chain, leading to a decrease in C-1-C-4 selectivities and an increase in C5+ selectivities with increasing CO conversion. The observed effects of Mn promotion on catalyst activity and product selectivity are discussed in terms of the mechanisms for CO hydrogenation and hydrocarbon chain growth. (C) 2012 Elsevier Inc. All rights reserved.