The Fischer-Tropsch synthesis was studied at 520 K on reduced iron catalysts at a pressure of 24 Pa in a continuous-flow fixed-bed reactor and at 9 kPa in a batch reactor. A quadrupole analyzer detector was used for continuous monitoring of the products formed. In the experiments at 24 Pa a preference for the formation of aromatics was observed. From this it is concluded that carbon adsorption on sites next to an occupied site is favoured as compared to adsorption on a clean part of the surface. In experiments at 9 kPa very little activity is observed for a period of about 15 ks, after which the customary activation sets in. However, when the catalyst is pretreated with a small quantity of oxygen, hydrocarbons up to C6 and C7 compounds are produced immediately. Consequently, the activation is also associated with the presence of oxygen on the catalyst. The activation is ascribed to the formation of certain active hydrogenation ensembles which form faster on the preoxidized catalyst. From preoxidation experiments with 18O2 it is concluded that the rate of dissociation of carbon monoxide on iron catalysts is a fast reaction compared to the overall Fischer-Tropsch reaction rate.