The methanation of CO2 as a relevant strategy for energy storage has been studied by operando X-ray absorption spectroscopy under dynamic H-2/CO2 and CO2 reaction atmospheres. A typical CO2 conversion of 81% was reached at 400 degrees C with a 23 wt.-% Ni/CaO-Al2O3 catalyst, yielding 80% of CH4. The operando XAS experiment under working conditions revealed pronounced structural changes, e.g., a fast bulk oxidation of the Ni particles after removal of H-2 from the H-2/CO2 (4:1) gas stream. A lower performance of the catalyst was observed in the subsequent methanation cycle due to the presence of a residual oxidized fraction of Ni. (C) 2015 Published by Elsevier Inc.