Potassium-promoted iron oxide dehydrogenation catalysts are the primary catalysts used for producing styrene. In this process, a large amount of steam is used and for economic reasons it is desirable to be able to operate at lower steam/ethyl benzene molar ratios without creating severe short-term deactivation. To address this issue, it is essential to understand how the catalyst deactivates. In this study, the stabilities of alpha-Fe2O3, K-Fe2O3 (10 wt% K+). KFeO2 (30 wt% K+), and a potassium polyferrite mixed phase (K2Fe10O16/K2Fe22O34) were investigated in different gas phases including H-2, CO2, and ethylbenzene. The effect of simultaneous steam addition was also considered. Thermogravimetric analysis and X-ray powder diffraction were used to monitor sample weight variation and phase change, respectively. alpha-Fe2O3 and K2Fe10O16/K2Fe22O34 were stable in CO2 but not stable in H-2. KFeO2 was resistant to H-2 but easily decomposed by CO2. K-Fe2O3 was adversely impacted by both H-2 and CO2. The results suggest that the reduction of the iron oxide in this system was mainly caused by surface deposited carbon instead of H-2. A transformation diagram is proposed for the phase changes of potassium-promoted iron oxide materials in the reaction-relevant gas phase conditions. (C) 2008 Elsevier B.V. All rights reserved.