We report on the high activity and stability of lanthanide oxysulfides, e.g., of La2O2S or Pr2O2S, as catalysts for the high temperature water-gas shift (WGS) and the reverse water gas shift (RWGS) reactions. The catalytic activity derives from a very large oxygen storage capacity based on the redox of the sulfur ion in the interconverting oxysulfide/oxysulfate phases during reaction. The WGS reaction lights off at around 400 degrees C and reaches equilibrium at temperatures >750 degrees C in tests conducted at high space velocities (similar to 14,000 h(-1)). No loss of sulfur or catalyst deactivation is observed with time-on-stream in sulfur-free gas mixtures at all temperatures. These catalysts have fivefold lower aerial reaction rates (mu mol/m(2)/s) than commercial Fe-Cr catalysts at 400 degrees C. However, unlike the iron oxide-based catalysts, they can be used up to 800 degrees C with high levels of H2S (700 ppm) present in the gas mixture. Inhibition by sulfur at lower temperatures was fully reversible. (C) 2011 Elsevier B.V. All rights reserved.