► Exposure of Li x Ni 1/3Mn 1/3Co 1/3O 2 to humidity delithiates a surface layer 10 nm thick. ► It is independent of the time of exposure ( t > few min) and of the cation disorder. ► It is associated to the formation of LiOH and Li 2CO 3 impurities at the surface. ► Lamellar compounds are more sensitive to humidity than olivines. LiNi 1/3Mn 1/3Co 1/3O 2 compound was successfully synthesized by the co-precipitation method. The effect of H 2O on LiNi 1/3Mn 1/3Co 1/3O 2 in humid atmosphere was investigated by structural, magnetic and electrochemical analysis, and Raman spectroscopy. The consequence is that immersion of LiNi 1/3Mn 1/3Co 1/3O 2 to H 2O and exposure of LiNi 1/3Mn 1/3Co 1/3O 2 to humid atmosphere (ambient atmosphere, 20 °C, 50% relative humidity) led to a rapid attack that manifests itself by the delithiation of the surface layer of the particles and the concomitant formation of LiOH and Li 2 CO 3 at the surface. This aging process occurred during the first few minutes, then it is saturated, and the thickness of the surface layer at saturation is 10 nm. After aging, an initial discharge capacity of 139 mAh/g was delivered at 1C-rate in the cut-off voltage of 3.0–4.3 V. About 95% of its initial capacity was retained after 30 cycles.