Proton nuclear magnetic resonance (H-1 NMR), diffusion-ordered NMR spectroscopy (DOSY-NMR), and ultraviolet-visible (UV-vis) spectroscopy techniques were used to study the specific intermolecular interactions within the binary mixtures of 1-butyl-3-methylimidazolium tetrafluoroburate, [bmim]BF4, and poly(ethylene glycol), PEG, with different average molecular weights (i.e., 200, 400, 600, and 1000) in the whole composition range. Also, the electrical conductivity and the viscosity of these four mixtures were measured at different compositions and temperatures. The results of 1H NMR spectra show that the change in proton chemical shifts of ionic liquid (IL) and PEG molecules in the mixtures is more than that of the pure ones. DOSY-NMR was used to measure the diffusion coefficients of the ionic species in the mixtures. Also, the other parameters such as hydrodynamic radii, transport numbers of anions and cations, and dissociation and association degrees of IL molecules were calculated from the diffusion coefficient and viscosity data. The overall results show that the behavior of the mixtures of [bmim]BF4 + PEG400, PEG600, and PEG1000 which show the synergistic effect on their viscosity is completely different from the IL + PEG200 mixture, and this difference can be interpreted well according to the special interactions occurring at the molecular level in these mixtures. Accordingly, in these mixtures, larger hydrodynamic radii and lower transport numbers of anions with respect to the cations may be attributed to the trapping of small anions in the polymeric clusters. Increasing the chain length and the concentration of PEG increases the association degree of the IL molecules due to increasing intermolecular interactions. The continuous decrement of electrical conductivity with increasing PEG concentration in these three mixtures confirms the cluster formation in the mixtures. The UV-vis spectra showed more hydrogen bonding interactions between unlike molecules in the mixtures containing higher molecular weight PEGs with respect to the lower ones.