Experimental densities and viscosities for c-butyrolactone (GBL) binary mixtures with 1,2,3-trialkyl-substituted imidazolium bis(trifluoromethylsulfonyl) imide ionic liquids, namely: 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl) imide [emmim][NTf2], 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl) imide [pmmim][NTf2] and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl) imide [bmmim][NTf2] (p = 0.1 MPa) in the temperature range from T = (293.15-323.15) K over the whole composition range. From the experimental density data the related excess molar volumes were calculated and fitted using Redlich-Kister's polynomial equation. The thermal expansion coefficients were also calculated from the acquired experimental density values. The influence of C-2 methyl group and effects of the alkyl side chain length of the 1,2,3-trialkylimidazolium ionic liquids on the nature of interactions and hydrogen bonding in the studied binary mixtures with GBL were discussed. Volumetric and transport properties of GBL binary mixtures with selected ILs are compared with available literature data for the corresponding 1,3-dialkyl-supstituted imidazolium based ionic liquids. (C) 2016 Elsevier Ltd.