The iron-based metal-organic-framework MIL53(Fe) has recently been tested as a cathode materials for Li-Ion batteries, leading to promising cycling life and rate capability. Despite a poor capacity of 70mAhg−1 associated with the exchange of almost 0.5Li/Fe, this result is the first evidence of a reversible lithium insertion never observed in a MOF system. In the present study, the MIL53(Fe) redox mechanism is investigated through first-principles DFT+U calculations. The results show that MIL53(Fe) is a weak antiferromagnetic charge transfer insulator at T =0K, with iron ions in the high-spin S =5/2 state. Its reactivity vs elemental lithium is then investigated as a function of lithium composition and distribution over the most probable Li-sites of the MOF structure. The redox mechanism is fully interpreted as a two-step insertion/conversion mechanism, associated with the stabilization of the Fe3+/Fe2+ mixed-valence state prior to the complete decomposition of the inorganic–organic interactions within the porous MOF architecture.