Industrial dehydrogenation of ethylbenzene to styrene is performed using potassium-promoted iron oxide catalyst. Many attempts have been made to understand the (deactivation mechanism of the catalyst based on the chemical differences between the fresh and used catalysts. In the present work. in addition. to the effect of chemical changes, the effect of structural changes of the internal areas on the catalyst activity was investigated. A fresh and used commercial catalyst from an industrial reactor which had been continuously used for two years under severe conditions (LHSV = 1 h(-1), T = 610 degrees C, mass ratio of steam to ethylbenzene = 1.2, P = 1.2 atm) was studied. Nitrogen adsorption, Hg porosimetry, X-ray diffractometer (XRD), scanning electron microscopy (SEM), X-ray fluorescence (XRF), FT-IR, Leco carbon analysis, and wet chemical analysis were performed on both fresh and used catalysts. The catalyst activity tests were performed in a lab-scale continuous fixed bed reactor, maintained at fixed temperature using an electrically heated furnace. It was found that micro and mesopores (d < 50 nm), that make 5% of the total catalyst porosity (21%). were completely filled with carbon deposits in the used catalyst (holding 1.1 wt.% carbon). This caused a surface area reduction of 29%, as measured by Brunauer-Emmett-Teller (BET) method. The remaining 95% of the pores were macropores that were slightly covered with carbon (holding only 0.6 wt.% carbon). Based on a variety of activity tests, it is proposed that the surface catalytic activity for ethylbenzene dehydrogenation follows the order: KFeO2 > Fe2O3/Fe3O4 > carbon (at 562 degrees C). At higher temperature of 639 degrees C, however, this trend reverses and carbon surface becomes more active than KFeO2 surface. (C) 2007 Elsevier B.V. All rights reserved.