Eight new functionalized guanidinium ILs based on small cations containing ether group (methoxyethyl group) or ester group (methyl acetate group) and TFSI- anion were synthesized and characterized. Physical and electrochemical properties of these products, including melting point, thermal stability. viscosity, conductivity and electrochemical window, were investigated. All the products were liquids at room temperature, and they had low-melting points. The viscosities of cg1(2o1)TFSI and cg2(2o1)TFSI were 46 and 48 mPa s at 25 degrees C, respectively. Electrochemical and thermal stabilities of these functionalized guanidinum ILs permitted them to become potential electrolytes used in electrochemical devices. (C) 2009 Elsevier Ltd. All rights reserved.