The dehydrogenation of 10,11-dihydro-5H-dibenz[b.f]azepine (iminodibenzyl) to 5H-dibenz[b.f] azepine (iminostilbene) was investigated over potassium-promoted iron oxide catalysts. The catalytic behaviour of the catalysts was found to be strongly influenced by the oxidation state of iron and by the amount of carbon deposits on the catalyst surface. Dilution of the iminodibenzyl feed by steam prevents reduction of the active iron oxide phase and reduces drastically the deposition of carbon, leading to high steady-state activities of potassium-promoted catalysts. Promotion of the iron oxides with 0.5 wt.-% potassium increases significantly the activity of the catalysts. Potassium improves the reducibility of iron oxides probably by enhancing the electron exchange between Fe3+ and Fe2+ ions, and results in an apparent increase of the number of sites active for the dehydrogenation of iminodibenzyl. Potassium carbonate and magnetite represent the main phases under reaction conditions in the presence of steam and are proposed as the active phases in the dehydrogenation of iminodibenzyl. Gasification of carbonaceous deposits by steam leads to the formation of carbon dioxide and potassium carbonate under normal reaction conditions. Therefore, the active centres are not sensitive to further carbon dioxide treatment.