Electrical conductivity of six binary liquid mixtures of 1,3-disubstituted and 1,2,3-trisubstituted imidazolium based ionic liquids (ILs) with bis(trifluoromethylsulfonyl)imide anion with c-butyrolactone (GBL) was measured at temperatures from (293.15 to 323.15) K and at atmospheric pressure (p = 0.1 MPa) over the whole composition range. The effects of the temperature and IL concentration on the electrical conductivity were systematically investigated. Values of the electrical conductivity were correlated using Casteel-Amis and Arrhenius equations. Molar conductivity was calculated and temperature dependance were fitted applying the Vogel-Fulcher-Tammann equation. Several semi-empirical models (quasi-Arrhenius and Bahe-Varela model) were also tested to correlate the electrical conductivity. The effect of the alkyl chain length and the influence of C-2 methylation of the imidazolium cation on the electrical conductivity features of six different binary mixtures were evaluated. (C) 2016 Elsevier Ltd.