Density, speed of sound, refractive index, dynamic viscosity, and surface tension measurements of 1-butyl-3-methylimidazolium methyl sulfate have been made as a function of temperature. The synthesis of the ionic liquid is given. The low viscosity of the ionic liquid suggests its use as a solvent in the extraction process for the separation of azeotropic mixtures. The thermal expansion coefficient of the ionic liquid was calculated from the density, and the results are discussed. An analysis of the influence of the alkyl chain length of the cation on the density was performed by comparison with recently published values.