In this work, we investigate the crystal chemistry of Fe/V-mixed NASICON Na 3 FeV(PO 4) 3 and Na 4 FeV(PO 4) 3 compositions structurally related to Na 3 V 2 (PO 4) 3 , a positive electrode for Naion batteries. To synthesize Na 4 FeV(PO 4) 3 , various synthesis routes (solid-state, sol-gel assisted, and electrochemical syntheses) were investigated. Direct syntheses resulted in forming a NASICON-type phase with the presence of NaFePO 4 and Na 3 PO 4 impurities. The successful preparation of pure Na 4 FeV(PO 4) 3 has been achieved by electrochemical sodiation of Na 3 FeV(PO 4) 3. Both synchrotron X-ray absorption and Mössbauer spectroscopies allowed probing the local V and Fe environments and their oxidation states in Na 3 FeV(PO 4) 3 and Na 4 FeV(PO 4) 3. Na 3 FeV(PO 4) 3 crystallizes in the space group C2/c (a = 15.1394(2) Å; b = 8.72550(12) Å; c = 21.6142(3) Å; β = 90.1744(9)°, and Z = 12), and it is isostructural to an ordered α-form of Na 3 M 2 (PO 4) 3 (M = Fe, V). It presents a superstructure due to Na + ordering, as confirmed with differential scanning calorimetry and in situ temperature X-ray diffraction. The electrochemically-sodiated Na 4 FeV(PO 4) 3 powder crystallizes in the space group R-3c (a = 8.94656(8) Å, c = 21.3054(3) Å, and Z = 6) within which the two sodium sites, Na(1) and Na(2), are almost fully occupied. Na 4 FeV(PO 4) 3 allows the electrochemical extraction of 2.76 Na + per formula unit within the voltage range of 1.3-4.3 V vs. Na + /Na through the Fe III/II , V IV/III , and V V/IV redox couples. This identifies an interesting material for Na-ion batteries.