Styrene is manufactured industrially through catalytic dehydrogenation of ethylbenzene on Fe-K oxide-based catalysts. It was invented by Sud-Chemie Group that the activity of the industrial ethylbenzene dehydrogenation catalysts (Styromax) based on the oxides of Fe and K is highly promoted by the addition of small amount (hundreds ppm-order) of precious metals such as Pd. The present work is intended to elucidate the role of Pd on the Fe-K catalyst empirically by use of a periodical pulse technique from a mechanistic point of view. The oxidative dehydrogenation was faster than the simple dehydrogenation, and it proceeded by consuming the surface lattice oxygen in the catalyst. The lattice oxygen was subsequently supplied from steam. Palladium added to the Fe-K oxide catalysts was found to enhance the rate of regeneration (supplying) of the lattice oxygen, although it hardly changed the rate of dehydrogenation of ethylbenzene or consumption of surface lattice O2- anions. This study demonstrated that steam works not only as a diluent but also as a reactant to form hydrogen and lattice oxygen.