As a follow-up to a previous alkali chloride poisoning study, the effect of up to 100 ppm KCl on the FischerTropsch synthesis (FTS) performance of representative iron (Fe-Si-Cu doped with Rb as the alkali) and cobalt (PtCo/Al2O3) catalysts was studied at 270 degrees C and 230 degrees C, respectively, by co-feeding KCl in a water/ethylene glycol (EG) solution. The used catalysts were characterized by XANES at the K and Cl K-edges; furthermore, ICP was used to analyze residual K and Cl ions possibly remaining in the FTS products. KCl was found to be a weak poison for the iron and cobalt catalysts. The addition of 20-100 ppm KCl deactivated the catalysts to only a low to moderate extent. For the cobalt catalyst, less than 25 ppm KCl was found to give negligible deactivation. The added KCl and EG-H2O solvent was found to slightly modify the selectivity for both catalysts, such that KCl slightly promoted light hydrocarbon formation as well as olefins and slightly suppressed C5+ and 2-olefin formation, while the EG-H2O solvent was found to have a different effect on the C-1-C-4 and C5+ selectivities. It appears that K and Cl played opposite roles in modifying hydrocarbon selectivities. The ICP results suggested 48-98% K and Cl ions were adsorbed by the iron catalyst. XANES results confirmed the presence of K and Cl ions on the used iron and cobalt catalysts and showed a structure with characteristics that were similar to bulk KCl. Two possible mechanisms, including site blocking by K and Cl ions and electronic modification impacting CO/H-2 adsorption, were proposed to explain the deactivating effect of KCl on the iron and cobalt catalysts.