Frequency-dependent shear viscosities of ammonium- and phosphonium-based ionic liquids with an oxyethylene chain were determined between 5 and 205 MHz, and compared with those of corresponding ionic liquids with an alkyl chain. The shear relaxation spectra show that the substitution of the alkyl chain with the oxyethylene one lowers shear viscosity through the reduction of relaxation time, which is attributed to the higher flexibility of the latter. Comparing the relaxation spectra of ionic liquids with those of non-ionic ones, higher shear viscosities of ionic liquids are ascribed to the larger high-frequency shear modulus due to the interionic Coulombic interaction. (C) 2011 Elsevier B. V. All rights reserved.