As a part of a research project on the production of iron carbides (Conejo, 1995), the precipitation of carbon from the gas phase was elucidated. It was found that neither iron oxides or metallic iron promote sooting but is due to the nucleation of a carbidic phase. The sequence of steps identified consist in the reduction of iron oxide into metallic iron, followed by a fast step of iron carbide formation and finally, if the carburizing agents remain in contact with the solid, the carbide phase completely disintegrates into a large mass dominated by carbon atoms (sooting). Some evidence was found on the stabilizing effect of sulfur on chi -carbide (Fe(5)C(2)).