Bifunctional metal-zeolite catalysts cobalt-ZSM-5 and cobalt-thoria-ZSM-5 have been examined for the conversion of synthesis gas (H 2/CO = 1) at 280 °C, 21 bar and WHSV = 0.77 g(g cat) −1 h −1 Thoria in amounts as low as 0.4 wt.% added to a cobalt-ZSM-5 with 9.0 wt.% cobalt caused a considerable increase in synthesis gas conversion and selectivity to gasolinerange hydrocarbons. Addition of thoria to the catalyst also resulted in a decrease of the aromatic fraction in the hydrocarbon product. Some Co-ThO 2-ZSM-B catalysts provided nearly 65 wt.% selectivity to gasolinerange product that contained 26 vol.% aromatics (alkylbenzenes). The selectivity to gasoline-range product exceeded the limit (48 wt.%) set by Schulz-Flory kinetics. Chemisorption and magnetization studies have been performed to determine metal particle size. Addition of thoria resulted in a decrease of the cobalt particle diameter from about 40 nm to 10 nm and caused an increase in CO/H adsorbed. The action of thoria as a promoter is possibly related to the presence of basic sites on its surface. Infrared studies have demonstrated a relative decrease in the number of Brönsted acid sites in ZSM-5 upon the addition of thoria, thus accounting for the decrease of the aromatic fraction in the liquid hydrocarbon product.