Densities of the ionic liquids [C(2)C(1)im]Cl, [C(3)C(1)im]Cl, [C(4)C(1)im]Cl, and [C6C1 im]Cl and their mixtures with ethanol were determined up to 40 MPa and 398 K. Density was modeled as a function of temperature, pressure, and composition using the Sanchez-Lacombe equation of state. Using the model, derived thermodynamic properties, namely isothermal compressibility, isobaric expansivity, and internal pressure, were calculated. This allowed for the estimation of the Hildebrand solubility parameters of these ionic liquids (ILs). Internal pressure was found to go through a maximum at low concentrations of ionic liquid in the case of [C(3)C(1)im] CI, [C(4)C(1)im] Cl, and [C6C1 im] Cl. These observations were interpreted in terms of a significant effect of the alkyl chain length on the interactions between the IL and the cosolvent, ethanol. It is speculated that this is in part due to possible clu stering between the anion (Cl-) and ethanol.