The dehydrogenation of ethylbenzene to styrene over a potassium-promoted iron oxide-based catalyst was studied with high time resolution using on-line mass spectroscopy. By means of transient kinetic experiments the initial stages of the dehydrogenation of ethylbenzene over the fresh catalyst and after regeneration were studied in detail. A very high degree of conversion and a very high yield of styrene were observed for both cases indicating clearly that the fully oxidized iron phases containing only Fe3+ ions (Fe2O3, KFeO2, K2Fe22O34) are responsible for high catalytic activity. The subsequently observed decrease in the yield of styrene (Y-ST) and the simultaneous increase in the concentration of hydrogen (C-H2) indicate that the dehydrogenation of ethylbenzene generating H-2 leads to the partial reduction of the only Fe3+-containing iron phases, and that this fast initial deactivation is mainly due to reduction. As a further consequence, the antiparallel course of Y-ST and CH2 observed again after regeneration shows that the treatment with steam had led to a partial re-oxidation of the near-surface regions of the catalyst from magnetite to Fe3+-containing oxides. It was confirmed that steam reacts with the coke deposits on the catalyst forming CO which is further converted by steam to CO2 (water gas shift reaction). However, the rather low concentrations of CO and CO2 observed during the regeneration, the achievement of quasi steady state under temperature-programmed reaction conditions and the absence of a hysteresis between the temperature-decreasing and -increasing branch indicate that coking does not significantly influence the catalytic activity on a short time scale using a high ratio of steam to ethylbenzene. (C) 2004 Elsevier B.V. All rights reserved.