A series of fluorohydrogenate ionic liquids (FHILs), Cat(+)(FH)(2.3)F(-) (Cat(+): alkylpyridinium cations and allylated quarternary cyclic ammonium cations), are prepared, and their physical and electrochemical properties are characterized. The viscosities and conductivities of alkylpyridinium-based FHILs increase and decrease, respectively, with an increase in the length of the side chain. The FHILs of allylated cations exhibit higher ionic conductivities than those of the corresponding propylated cations and even those of the ethylated cations in some cases, which is explained by differences in volume, charge distribution, and rotational freedom of the side chains according to quantum mechanical calculations. The electrochemical windows of the FHILs are around 4 V.