A new series of low-melting, low-viscosity, hydrophobic ionic liquids based on relatively small tertiary sulfonium cations ([R1R2R3S]+, wherein R1, R2, R3 =CH3 or C2H5, R3 =CH2CH2OCH3, CH2CH2COOCH3, CH2CH2CN) and bis(fluorosulfonyl)imide (FSI−) anion have been prepared and characterized. The important physicochemical and electrochemical properties of these salts, such as melting point, glass transition, viscosity, density, ionic conductivity, thermal and electrochemical stability, have been determined. The influence of structure variation in the tertiary sulfonium cations on the above physicochemical properties is discussed. Among these new salts, some of them show the desirable properties including low-melting points, low viscosities, and high conductivities, to be selected as potential candidates as electrolytes in energy devices, and two salts are ionic plastic crystals.