Details of the reaction mechanism of ethylbenzene (EBDH) dehydrogenation were investigated using kinetic analyses of highly active and stable La0.8Ba0.2Fe0.4Mn0.6O3-delta (LBFMO) perovskite oxide catalyst with a quadrupole mass spectrometer by measuring the instantaneous behavior. Under EBDH with a steam condition, the stoichiometric factor of oxygen in LBFMO catalyst in the steady-state reaction was found to be 2.927, which is lower than 3. The catalyst worked in the reduced state in which a few layers of lattice oxygen were consumed, supported by X-ray photoelectron spectroscopic analyses. Additionally, the reactive lattice oxygen and vacancy were involved with the reduction-oxidation (redox) mechanism of EBDH with steam. The respective amounts were 17.0 mmol mol-cat(-1) for available lattice oxygen and 68.5 mmol mol-cat(-1) for vacancy over LBFMO under the steady state condition. LBFMO catalyst showed high and stable EBDH activity by virtue of the redox mechanism using this lattice oxygen and vacancy. (C) 2013 Elsevier B.V. All rights reserved.