The thermophysical properties of ionic liquids (ILs) can be judiciously modulated by adding suitable co-solvents to make them an economical alternative for various industrial processes. In our search for the better thermophysical properties of these neoteric solvents, herein, we had investigated the thermophysical, acoustic, and refractive properties of imidazolium-based ILs with the bromide counter ion and alkyl chain from butyl to octyl when they are mixed with PEG-600. The density (ρ), speeds of sound (u), and refractive index (n D ) for the pure components of the mixtures and their binary mixture were measured at their whole composition range, and temperature ranges from 293.15 to 323.15 K and at atmospheric pressure. These properties were used to calculate the excess molar volume (V m E ), excess molar isentropic compressibility (K s,m E ), and deviation in refractive index (Δ ϕ n D ) using known relations. These properties were then used to interpret the solute−solute and solute−solvent interactions present in the studied mixtures and to understand the non-ideality within the mixture. Several mixing rules were used to calculate mixtures' refractive indices and correlate with the experimentally measured values. We had further predicted the V m E values of the studied mixtures using the well-known Prigogine−Flory−Patterson theory and extended real association (ERAS) model. Results discussed in this manuscript could help us gain insights to fine-tune the properties of the ILs in a more better and balanced way for several industrial applications.