Densities, viscosities, and electrical conductivities were determined and discussed for a binary mixture containing 1-butyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide ionic liquid, bmpyrr­NTf2, and diethyl carbonate (DEC) as a function of temperature (T = 293.15–323.15 K) at atmospheric pressure (p = 0.1 MPa) over the entire range composition. Based on experimental density data, the related excess molar volumes were calculated and fitted using Redlich–Kister’s polynomial equation. The Casteel–Amis equation was used to correlate data obtained from electrical conductivity measurements. The effect of viscosity on electrical conductivity was discussed by the Walden rule. The Bahe–Varela theory, derived from the the pseudo-lattice model, has been successfully applied to explain the influence of the mixture composition on electrical conductivity. Based on physicochemical characterization supported with computational simulations for binary mixtures (bmpyrr­NTf2 + DEC), the interactions between these components were discussed.