The density, viscosity, and refractive index of a pure asymmetrical dicationic ionic liquid, 1-(1-methypiperidinium-1-yl) pentane-(1-pyridinium) bis(trifluoromethane)sulfonamide, and its binary mixtures with acetonitrile were investigated at temperatures from 293.15 K to 323.15 K over the entire range of composition at ambient pressure. From the experimental data, the excess molar volumes, dynamic viscosity deviations, and refractive index deviations were, respectively,calculated and correlated with Redlich-Kister-type polynomials. The temperature dependence of viscosity for pure ionic liquid was described by modified Vogel.Tammann-Fulcher (VTF) equation. The enthalpy and entropy of activation for viscous flow were obtained. The values of molar refraction and polarizability indicated that the dominant interactions between ionic liquids and acetonitrile were dipole-dipole molecular interactions. These thermodynamic properties were discussed in terms of nature of molecular interactions between the ionic liquid and acetonitrile.