The characterization with Mossbauer absorption spectroscopy of supported iron oxide-based catalysts, containing only iron oxide or containing both iron oxide and potassium carbonate, after preparation as well as after exposure to the reaction conditions used in I-butene dehydrogenation is described. The fresh catalysts all contain well-dispersed iron(III) oxide particles. The potassium carbonate compound remaining on the surface after decomposition of KFeO2 formed during the calcination step disperses the iron oxide phase in the case of the magnesia and zirconia supports. When using titania as a support the formation of a mixed oxide is observed. With the Fe/TiO2 catalyst FeTiO3 has been formed after dehydrogenation. In the Fe/MgO catalyst next to a surface-stabilized Fe1-xO phase, well-dispersed Fe3+ species are present. In the zirconia-supported sample Fe3O4 is detected. When both iron oxide and potassium carbonate are present, the iron-containing phase in the K,Fe/MgO system also consists of a mixture of the Fe1-xO phase and well-dispersed Fe3+ species. In the K,Fe/ZrO2 catalyst besides the Fe3O4 phase a well-dispersed Fe3+ species is observed. However, the magnetite crystallites in this catalyst are notably smaller than in the Fe/ZrO2 catalyst after dehydrogenation. No distinction between a well-dispersed (alpha)Fe2O3 or a well-dispersed KFeO2 phase can be made at this stage. It is most probable that the main functions of the potassium in the supported catalysts can be described as, firstly, dispersing the iron phase, and secondly, providing the, gasifying properties required for the auto-regenerative character of the catalyst.