The impact of air exposure on LiFePO4-C nanocomposites has been investigated at moderate temperature. We show here that the storage in 120 degrees C hot air for 30 days leads not only to the material delithiation but also to the formation of an amorphous ferric phosphate side-phase, accounting for 38% of the total iron. The formed phase is found to be partially hydrated, suggesting a water-driven aging mechanism and a proposed hypothetic formula: LixFePO4(OH)(x). The side-phase displays new electrochemical activity but poor cyclability and the overall battery performance is thus deteriorated. The regeneration of pristine structure, together with the performance recovery can be achieved by a simple thermal treatment under inert atmosphere. (C) 2009 Elsevier B.V. All rights reserved.