The current study focuses on several phosphonium-based ionic liquids, namely, trihexyltetradecylphosphonium chloride, trihexyltetradecylphosphonium acetate, and trihexyltetradecylphosphonium bis{(trifluoromethyl)sulfonyl}amide. The objective was to study the influence of pressure as well as that of the anion on several properties of this type of ionic liquids. Densities in pure ionic liquids as a function of temperature and pressure have been determined. Other thermodynamic properties, such as the isothermal compressibility, the isobaric expansivity, and the thermal pressure coefficient, have been calculated. Density measurements have been performed at a broad range of temperature (298 < T/K < 333) and pressure (0.1 < p/MPa < 65) using a vibrating tube densimeter. A simple ideal-volume model was employed for the prediction of the molar volumes of the phosphonia at ambient conditions, which proved to compare favorably with the experimental results.