Imidazolium-based ionic liquids (ILs) have been widely investigated in the biofuels process. In this work, the experimental densities and viscosities of binary mixtures of IL, 1-octyl-3-methylimidazolium chloride, with aprotic cosolvents (i.e., N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and pyridine) were studied at temperatures ranging from (303.15 to 353.15) K at atmospheric pressure. The Vogel−Fulcher−Tammann equation is used to correlate the viscosity data. Toward further understanding the influences of cosolvents on the properties of IL, the excess properties of density and viscosity deviations are calculated as well as the energy barrier. The effects of cosolvents on the density and viscosity are discussed.