The activity of Ru/TiO2 and TiO2-promoted Ru/SiO2 catalysts for Fischer-Tropsch synthesis has been investigated. The Ru dispersion of these catalysts was determined by TEM, and the fraction of the Ru surface available for H-2 adsorption was determined by H-1 NMR. Even after low-temperature reduction, the surface of titania-containing catalysts is covered by titania to a substantial degree. The specific activity and selectivity of these catalysts are dominated by the interactions occurring between the Ru particles and the titania overlayer. As the fraction of the Ru particle surface covered by titania increases, the turnover frequency for CO consumption passes through a maximum, while that for methane formation decreases monotonically. The probability for chain growth and the olefin-to-paraffin ratio of the products increase with increasing titania coverage. These trends are attributed to the effects of the titania overlayer on the catalytic properties of Ru.