A series of new polyhedral oligomeric silsesquioxane ionic liquids (POSS-ILs) based on the trimethylpropylammonium dielectric constants, which, conversely, display slight differences originating from the nature of the anions. The long alkyl-chain substituents on the POSS core infer on this series of ILs an overall hydrophobic character and solution properties comparable to those of classic cationic surfactants, resulting in an effective extractant capability of polyanions from aqueous solutions. Their solution diffusion properties, as investigated by room-temperature dynamic light scattering, solution conductivity, density, and osmometric measurements, do not provide evidence for the occurrence of any hepta(isooctyl)octasilsesquioxane cation and a variety of anions have been synthesized and characterized. Their thermal behavior has been ascertained by thermogravimetric and differential thermal analyses and differential scanning calorimetry; room temperature conductivity, dielectric constants, and dynamic contact angles have been investigated as well. The obtained data show that the presence of the POSS moiety is responsible not only for their thermal properties, but also for their low room-temperature conductivity and significant self-assembling processes.