The equilibrium potentials for Sn–Na alloy formation reactions are investigated in an intermediate temperature ionic liquid, NaFSA–KFSA (x NaFSA = 0.56, x KFSA = 0.44, FSA = bis(fluorosulfonyl)amides), at 363 K. Galvanostatic intermittent titration technique (GITT) is employed to obtain the potential–Na composition curve of the Sn–Na alloy. Ex situ X-ray diffraction (XRD) analysis reveals that plateaus at 0.16 and 0.08 V vs. Na+/Na in the potential–Na composition curve correspond to α-NaSn/Na9Sn4 and Na9Sn4/Na15Sn4 two-phase states, respectively. Although distinct potential plateaus are not observed, the equilibrium potentials of β-Sn/NaSn2 and NaSn2/α-NaSn are suggested to be 0.53 and 0.46 V vs. Na+/Na, respectively. Finally, the Gibbs free energies of formation of the Sn–Na alloys are calculated and compared with reported data.