The effect of cationic and anionic chain lengths and temperature dependence of physical properties for a new series of 1-butyl-3-propanenitrile and 1-decyl-3-propanenitrile imidazolium-based ionic liquids were studied in a broad range of temperature (293.15 to 353.15) K at atmospheric pressure. The present ILs show lower densities, higher viscosities, and comparable values of refractive index compared to those of analogous nitrile-functionalized ILs with different anions. The thermal expansion coefficient, compressibility coefficient, and molar refraction were calculated from the experimental values of density and refractive index. These ILs show a weak temperature dependency of the thermal expansion coefficient. © 2014 American Chemical Society.