Abstract For ex-choride Ru catalysts supported on high surface area graphite and promoted by potassium oxide, the influence of the reduction temperature on the surface properties and on the catalytic performance in the Fischer-Tropsch reaction (1bar, H2/CO=2/1) is investigated. The Ru metallic particle size generated on these catalysts was studied by CO chemisorption and checked by TEM. Characterization of the catalysts by XPS analysis and microcalorimetry of CO adsorption reveals the presence of residual chloride anions after reduction (hydrogen flow) at 573 and even 673K. The chloride traces neutralize the effects of the potassium promoter in the Fischer-Tropsch reaction. When catalysts are reduced at 773K, lower methane selectivity and higher olefin/paraffin ratio are observed for those with potassium added. Our results show the significance of the reduction treatment and the presence of residual chloride anions to achieve a ruthenium based Fischer-Tropsch catalyst with improved selectivity to light olefins.