Promoting a precipitated FeCuSiO2 catalyst with Mn has been shown to significantly improve its catalytic activity for Fischer-Tropsch synthesis (M). Although the impact of K promotion on the activity of Fe catalysts with and without Mn addition has been studied previously, no one has previously delineated how K influences the concentration of active surface intermediates and the intrinsic site activities of Fe and, more specifically, Mn-promoted Fe catalysts. This paper addresses that issue using steady-state isotopic transient kinetic analysis (SSITKA). Adding K at relatively low concentrations to the base Fe and Mn-promoted Fe catalysts improved the catalyst activity, but the activity of the catalysts declined with the addition of an excess amount. The percentage of light olefins (C-2-C-4 fraction) and chain growth probability (a) were enhanced, as expected with the presence of K, regardless of Mn addition. The addition of K decreased the BET surface area and the concentration of surface exposed Fen atoms (as determined by CO chemisorption). The intrinsic site activities (TOFITK) of all of the Fe catalysts determined using SSITKA were essentially identical, regardless of the concentration of added K or Mn promotion. This indicates that adding K to unpromoted or Mn-promoted Fe catalysts did not greatly affect the activity of the active sites. Rather, the higher catalyst Activities observed for the Fe and Mn-promoted Fe catalysts with K addition were due primarily to an increase in the number of active surface intermediates leading to hydrocarbon products. (C) 2008 Elsevier Inc. All rights reserved.