Details of the surface structure and composition of potassium-promoted iron oxide model catalyst films prepared on Ru(0001) are investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). At 700 K, polycrystalline KFeO2 films are formed. On annealing at the temperature of the technical dehydrogenation reaction (870 K), the potassium content in KFeO2 films decreases and a mixed phase which consists of polycrystalline KFeO2 on top of partially uncovered KxFe22O34(0001) forms. Both phases are exposed at the surface. At 970 K, the whole film transforms to KxFe22O34(0001). Similarities with technical catalyst samples suggest that the pressure gap is bridged, and for the first time, atomic details of the potentially catalytically active surface phase are presented.