An overview of experimental and theoretical studies recently performed in the Oeiras/Lisbon laboratories is provided. Typical showcase examples of UCST demixing of ionic liquid solutions are presented and discussed. Co-solvency, pressure, and isotope effects were investigated. In order to rationalize the observed effects, a phenomenological g(E)-model was successfully applied, which permitted us to establish strong links between phase behavior and excess properties. Speed of propagation of ultrasound waves and densities in pure ILs as a function of temperature and pressure were determined from which several other thermodynamic properties such as compressibilities, expansivities and heat capacities, were derived. The quasi-ideality of mixtures of ILs, as judged by the small values of their excess volumes, could have been predicted by the master linear representation of their pure liquid volumes as the size of either the cation or the anion change. Research has been carried out at a broad range of pressures, typically up to 1600 bar, sometimes inside the metastable liquid region. The current study focuses on [C(n)mim][PF6], [C(n)mim][NTf2], and [C(n)mim][BF4] where n is usually 4, but generally 2 <n <10.