Sodium-based layered oxides undergo several structural changes upon the (de-)sodiation process and reveal a strong tendency towards water intercalation at lower sodium contents. However, valuable information about their handling during the investigation are still rare. Herein, we report an investigation of the water sensitivity of layered Na_xNi_0.22Co_0.11Mn_0.66O₂ with mixed P2/P3 structure via X-ray diffraction. At lower sodium contents, i.e. below x ≈ 0.33 or above 3.6 V vs. Na/Na⁺, a strong tendency for the uptake of water is observed, supported by the appearance of new diffraction peaks confirming a dramatically increased interlayer distance. However, at high sodium content (x > 0.33) the material can be processed in open air without major changes.