We show that the fuel additive ferrocene leads to the formation of condensation nuclei in the combustion zone prior to the formation of carbonaceous particles. The condensation nuclei most likely are iron oxides. The carbonaceous matter preferentially condenses at the surface of the iron oxide nuclei where it is burnt in the last stage of the combustion. Similar results are obtained if palladium aerosol instead of ferrocene vapor is added to the fuel. Under the ideal conditions of a laminar acetylene flame diluted with argon and seeded with ferrocene almost complete burnout of the carbonaceous matter is achieved. In turbo-charged heavy duty diesel construction engines, the same basic phenomena are observed, yet the burnout is Far from complete.