Transient kinetic regimes of Fischer-Tropsch (FT) synthesis with a potassium-promoted iron catalyst have been observed and characterized by time-resolved conversion and selectivity studies using H-2/CO and a H-2/CO2 synthesis gases. Up to six episodes relating to catalyst transformations/reconstructions could be distinguished. Amazingly, with the H-2/CO2 synthesis gas a Fischer-Tropsch product of nearly the same composition as that with the H-2/CO synthesis gas was finally obtained at the steady state. However, the transient episodes lasted for a long time. Selectivity has been related to steps of elemental reactions by use of a kinetic model. As intrinsic FT feature, the principle of selective inhibition is established fron the detailed selectivity results. Spatial constraints at the FT sites appear also to control selectivity The iron catalyst exhibits fundamental differences in how generating FT sites, as compared with cobalt catalysts. FT sites on iron are stable, whereas FT sites on cobalt are of dynamic nature.