In the present work, a series of 1-propyronitrile-3-alkylimidazolium bromide ionic liquids (C2CN CnimBr, where n = 4, 6, 8, and 10) were synthesized and characterized using Fourier transform infrared spectroscopy (FTIR), NMR, and elemental analysis. Physical properties such as density, viscosity, and refractive index are measured and reported for a temperature range of (293.15 to 353.15) K and at atmospheric pressure. The influence of the alkyl chain on these properties is discussed. The present synthesized ionic liquids show a weak temperature dependency on the thermal expansion coefficient. © 2010 American Chemical Society.