This work provides a study based on acyclic and cyclic sulfonium ionic liquids (ILs) with alkyl and ether-functionality on the cation paired with the bis{(trifluoromethyl)sulfonyl}imide, [TFSI] À , or the bis(fluorosulfonyl)imide, [FSI] À , as the counter anion. Herein, thermophysical characterisation of nine sulfonium-based ILs concerning the density, viscosity and conductiv-ity and thermal properties including phase transition behaviour and decomposition temperature is reported. The electrochemical stability of the ILs was also measured by cyclic voltammetry at a glassy carbon macro-disk electrode. All of the ILs showed low melting point, low viscosity and good conductivity and could serve as potential electrolytes for energy storage devices.[a] S.