Monoclinic Fe-2(MoO4)(3 )(FMO) shows distinct structural and electrochemical differences in the intercalation mechanism, depending on the guest ion.(1,2) FMO undergoes a single-phase reaction in a Na-ion cell, but a two-phase reaction in a Li-ion cell. Attempts to understand the difference in the mechanisms have been hindered by a lack of structural information on the fully sodiated phase Na2Fe2(MoO4)(3 ) due to its structural complexity and the unavailability of a single crystal. In this work, we have solved and refined the crystal structure of Na2Fe2(MoO4)(3 ) for the first time, using the technique of ab initio structure determination from powder diffraction data. Along with electrochemical and structural characterization, 3D bond valence sum difference map calculations enabled us to ascertain the decisive factors that determine such differences, in terms of the interatomic distance and coordination environment of a guest ion. In the case of Na insertion, only a slight expansion of the structure makes the cavity sites of FMO suitable for Na ions, with adequate distances and coordination with surrounding oxygen atoms, resulting in a solid-solution-type single-phase reaction. In the case of Li insertion, the cavity sites are so large for a Li ion that a significant structural change involving tilting of the FeO6 and MoO4 polyhedra is required to accommodate the Li-ion in a suitable local environment, which does not allow a continuous structural change but results in a two-phase reaction.