The catalytic dehydrogenation of 10,11-dihydro-5H-dibenz[b.f]azepine(iminodibenzyl) to 5H-dibenz[b.f]azepine (iminostilbene) has been investigated over potassium-promoted iron oxide catalysts in the presence of steam. The reaction was performed at 550-600-degrees-C and steam/iminodibenzyl ratios of 50-300 mol/mol. Under the conditions used selectiviies to iminostilbene up to 80% at nearly 100% iminodibenzyl conversion were achieved. The observed overall reaction behaviour was influenced by contributions from heterogeneous (catalytic) and homogeneous (non-catalytic) reactions. The homogeneous reaction pathway became prominent at high space time factors (W(cat)/F(IDB) > 2 g(cat) h/g(IDB) and/or high temperatures above 600-degrees-C. Potassium-promoted catalysts (7 wt.-% K) prepared from different iron oxide phases, hematite, maghemite and magnetite, showed all similar selectivities to iminostilbene, indicating that the choice of the iron oxide precursor had no significant influence on the selectivity of the final potassium-promoted iron oxide catalyst. The activity of the catalysts increased with higher BET surface area. The behaviour of all catalysts was strongly influenced by the steam/iminodibenzyl ratio in the feed. Both selectivity to iminostilbene and conversion of iminodibenzyl increased with higher steam/iminodibenzyl ratios. The different roles of steam in the catalytic reaction are discussed.