1,3-Dimethylimidazolium bis(fluorosulfonyl)amide, [C(1)mim]FSA, is an ionic liquid with a lower viscosity than the bis(trifluoromethanesulfonyl)amide salt, [C(1)mim]NTf2, but with a higher melting point. The single-crystal X-ray structure analysis reveals that this is because the [C(1)mim]FSA crystal has low coordination ion number and three types of highly symmetric polydentate C-H center dot center dot center dot O hydrogen bonds.