The synthesis of hydrocarbons from synthesis gas was studied over a series of zeolites (HX, HY and CuX) mixed with Ru/TiO2 catalysts at 1 atm, 473-573 K, H2:CO=1 and GHSV = 1800 h-1. A Ru/TiO2SiO2 catalyst was used for comparision. It was found that the acidity of the zeolites could modify the catalytic properties by (1) increasing CO conversion, (2) increasing C4 hydrocarbon formation, (3) increasing the selectivites for olefins and (4) increasing isobutene formation. The results can be interpreted with a bifunctional mechanism of the catalysts. In the addition, the rate-determining step is shifted from the reactions taking place over the Fischer-Tropsch catalysts to diffusion of C4 hydrocarbons into the zeolites.