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Sodium transition metal silicates are potential candidate electrode materials to enable two-electron redox per transition metal ion center. Yet, the electrochemical reaction mechanism remains elusive despite the widely reported electrochemical activity for this class of materials as intercalation cathodes for Na-ion batteries. Adopting monoclinic Na 2 CoSiO 4 as a model compound, we used high-resolution synchrotron X-ray diffraction (XRD) and X-ray pair distribution function (PDF) analysis to elucidate the structure of the partially desodiated Na 2-x CoSiO 4 phases for the Co 3+ /Co 2+ redox couple. The appearance of satellite reflections in the intermediate Na 1.5 CoSiO 4 and NaCoSiO 4 phases manifests the formation of modulated structures, which are induced by Na + /vacancy and Co 2+ /Co 3+ charge orderings. Accounting for these structural orderings is important to understand the function and performance of sodium transition metal silicate electrodes.