The effect of halides and potassium addition on cobalt Fischer-Tropsch synthesis (FTS) was studied using 1L CSTR at 493 K, 1.99 MPa and H-2/CO of 2.0. The potential catalyst poison was co-fed with syngas after reaching steady-state FTS. Halides were added in an aqueous solution of HX (HF, HCl, HBr) and potassium as aqueous potassium nitrate solution along with syngas at various concentration levels ranging from 500 ppbv to 100 ppmv. Fluoride addition did not affect FT performance of cobalt up to 1.0 ppmv level. However, the presence of other halides (Cl- and Br-) decreased the FT activity proportionally. Addition of potassium decreased cobalt FT activity considerably and increases syngas conversion to methane and CO2 at higher potassium concentrations. The observed poisoning effects for metallic cobalt could be attributed to a blocking effect of cobalt metal sites by halides and potassium in agreement with results of H-2 chemisorption on used cobalt FT catalysts.