Funding: The authors want to thank EPSRC (EP/R030472/1) and the Faraday Institution (FIRG018) for their financial support. In addition, AGM wishes to thank the Faraday Institution for financial support and training (Grant number FITG033). The authors also thank EPSRC Light Element Analysis Facility Grant EP/T019298/1 and the EPSRC Strategic Equipment Resource Grant EP/R023751/1. The iron-based polyanionic fluoro-oxalate material, KFe(C2O4)F (KFCF), has been synthesized by hydrothermal methods. This compound shows promising reversible lithium and sodium insertion properties as a cathode material. The material delivered a first-cycle discharge capacity of 120 mAh g-1 at ∼3.3 V (Li+/Li) and 97.4 mAh g-1 at ∼3.0 V (Na+/Na) in LIB and NIB, respectively. Stable cycling performance was observed in both cases. The involvement of reversible Fe2+/Fe3+ redox was confirmed by ex-situ Mössbauer spectroscopy supported by first-principles calculations. This study reveals promising performance from a mixed oxalate-fluoride based polyanionic material thereby opening up further possibilities for materials discovery in the design of new electrode materials. Publisher PDF