The binary mixture of N-butylpyridinium tetrafluoroborate ([bpy][BF4]) and 4-methyl-N-butylpyridinium bis(trifluoromethylsulfonyl)imide ([4bmpy][Tf2N]) ionic liquids (ILs) has recently been proposed as an environmentally friendly solvent in the liquid-liquid extraction of toluene from heptane. In addition to the extractive properties, a solvent must possess adequate physical properties to be used in an industrial process. In this paper, we have measured refractive indices, densities, and dynamic viscosities of {[bpy][BF4] + [4bmpy] [Tf2N]} IL mixtures over the whole range of compositions at temperatures from 293.15 K to 353.15 K. A comparative analysis between physical properties of binary IL mixtures and those of sulfolane has been made. The accuracy of the group contribution method proposed by Ye and Shreeve in predicting experimental densities of a binary pyridinium-based IL mixture composed of four different ions has been evaluated. The Bingham and the Grunberg-Nissan mixing rules have also been used to estimate the viscosities of the binary IL mixtures.