The stability of [C2C1im][FSA] (C2C1im+: 1-ethyl-3-methylimidazolium; FSA–: bis(fluorosulfonyl)amide) and [C2C1im][TFSA] (TFSA–: bis(trifluoromethylsulfonyl)amide) ionic liquids against Na metal has been investigated in view of their application as electrolytes for Na secondary batteries. Cyclic voltammetry revealed that Na metal electrodeposition/dissolution reactions occur in Na[FSA]–[C2C1im][FSA], whereas these processes do not occur in Na[TFSA]–[C2C1im][TFSA]. Both visual and spectroscopic changes were observed for Na[TFSA]–[C2C1im][TFSA] after immersion of Na metal for 4 weeks, but no changes were observed for Na[FSA]–[C2C1im][FSA]. X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy indicated that there were differences in the thickness of the surface films on Na metal immersed in these ionic liquids. The presence of Na+ also affects the thickness of the surface film, and the nature of the surface films determines the difference in the stability of Na metal in these two ionic...