The changes appearing for LiFePO"4-C nano-composites exposed to atmosphere at 120^oC have been structurally and chemically examined by the use of TGA, XRD, XPS, Mossbauer, ^7Li MAS NMR and electrochemical methods. The results conclude that a highly disordered phase resulting from the aging of LiFePO"4 appears on the surface of the grains of the material, is assigned to a phosphate phase and can insert lithium around 2.6V with poor reversibility. The essential role of water has been investigated and clearly demonstrated. Thus, the aging mechanism occurring in hot humid air is completely different from a simple oxidation as well as from the aging process observed above 150^oC and involves the incorporation of hydroxyl groups. In addition, Fe"2O"3 formation has not been observed for such an aging in mild conditions.