The effects of added potassium (0-2 wt%) on the catalytic properties of iron supported on activated carbon were studied for the Fischer-Tropsch (FT) reaction to hydrocarbons and oxygenates. Activities for both FT and water-gas shift appear to increase with intermediate amounts of potassium, but decrease for larger amounts. Potassium significantly modifies selectivites of the products, shifting them to higher-molecular-weight hyddrocarbons and alcohols. The relative nature of branched paraffins and linear paraffins, and internal olefins and linear olefins also change.