The types of carbonaceous species formed on a precipitated iron catalyst by interaction with carbon monoxide and synthesis gas have been studied by isothermal and temperature programmed reduction of the deposited species with hydrogen. It has been found that by (dissociative) adsorption of carbon monoxide on a freshly reduced catalyst, a type of carbonaceous material is formed that is located in subsurface layers. By CO adsorption on a catalyst that has been used previously in a deposition/reduction experiment, a very reactive carbon species is formed. Upon hydrogenation, this species not only forms methane but also ethane. Hydrogenation of carbonaceous species deposited during the synthesis shows that the same highly reactive species is present on the active catalyst, indicating that this species is the active surface species in the synthesis.