Organic compounds represent an appealing group of electrode materials for rechargeable batteries due to their merits of biomass, sustainability, environmental friendliness, and processability. Disodium terephthalate (Na₂C₈H₄O₄, Na₂TP), an organic salt with a theoretical capacity of 255 mAh·g⁻¹, is electroactive towards both lithium and sodium. However, its electrochemical energy storage (EES) process has not been directly observed via in situ characterization techniques and the underlying mechanisms are still under debate. Herein, in situ Raman spectroscopy was employed to track the de/lithiation and de/sodiation processes of Na₂TP. The appearance and then disappearance of the –COOLi Raman band at 1625 cm⁻¹ during the de/lithiation, and the increase and then decrease of the –COONa Raman band at 1615 cm⁻¹ during the de/sodiation processes of Na₂TP elucidate the one-step with the 2Li⁺ or 2Na⁺ transfer mechanism. We also found that the inferior cycling stability of Na₂TP as an anode for sodium-ion batteries (SIBs) than lithium-ion batteries (LIBs) could be due to the larger ion radium of Na⁺ than Li⁺, which results in larger steric resistance and polarization during EES. The Na₂TP, therefore, shows greater changes in spectra during de/sodiation than de/lithiation. We expect that our findings could provide a reference for the rational design of organic compounds for EES.