Iron-catalyzed tar decomposition is studied by inspecting changes in catalytic activity and its correlation to the metal oxidation numbers. A carbon-aerogel-supported iron (Fe/CAG) catalyst was used herein to decompose toluene (model tar) at between 500 degrees C and 700 degrees C. The catalytic decomposition was studied in a conventional reaction system, and the catalyst was characterized prior to and after the reaction to understand the effect of the structural changes in the catalytic activity. The results demonstrated that the iron oxidation state and coke deposition strongly depend on the reaction temperature. The X-ray diffraction patterns confirmed that at T < 700 degrees C, several Fei+ are present, while at 700 degrees C, Fe prevails, suggesting a reconstruction of the metal clusters on the surface. The Fe/C ratio determined by elemental mapping (EDS), as well as thermogravimetric analysis coupled to mass spectrometry (TGA-MS) of the spent catalysts, confirmed that this surface reconstruction is related to carbothermic reduction of the metal oxides and assisted by carbon deposition.