The effect of alkali impurities on Co-based Fischer-Tropsch (FT) catalysts is important for processing biomass-derived synthesis gas containing inorganic ash impurities. The effects of Na, K, Li, and Ca impurities on gamma-Al2O3-supported Co-Re powder catalysts were studied at impurity loadings between 25 and 1,000 ppm. Impurity addition did not have any effect on H-2 chemisorption, but the catalyst activity decreased during FT synthesis experiments. The impurities were also found to slightly increase the reduction temperatures of Co. Carbon selectivities to CH4 decreased with increasing impurity loading, while CO2 and C5+ hydrocarbon selectivities increased. Catalyst behavior was attributed mostly to electronic effects from the alkali addition, leading to decreased surface H concentrations and increased CO adsorption and dissociation.