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Sintering of nickel particles is a well-known path of deactivation for Ni/Al₂O₃ catalysts. Considering the CO₂ methanation in the context of Power-to-Gas, a sintering study for up to 300 h was performed in a controlled atmosphere between 450 and 600 °C. Since water is a product of the methanation reaction and is known to favor the particle sintering, the H₂O:H₂ molar ratio was varied in the range 0–3.2. Characterization of the post mortem samples showed sintering of both nickel and support particles. The absence of carbon oxides in the gas feed allows us to rule out other causes of deactivation such as carbon deposits. A sintering law is derived from the loss of metallic surface area with time-on-stream according to local temperature and H₂O:H₂ molar ratio. An excellent fit of the experimental data was obtained allowing the prediction of the metallic surface area within 15%.