The liquid–liquid equilibrium of the [C4mim][NTf2]–1-butanol system presents upper critical solution temperature (UCST) behaviour. We report the influence of added water as well as the effect of hydrostatic pressure on the cloud points. Similarly to previously studied systems that involved RTILs in our laboratories, water is shown to be a very good co-solvent with alcohols. If a small amount of water is present in the [C4mim][NTf2]–1-butanol system it decreases the UCST as much as 1.5 K per mol% of water added to butanol. The hydrostatic pressure effect on cloud points is rather modest (≈−3.5 × 10−3 K bar−1). As for liquid–vapour equilibria, a new apparatus which employs a high-pressure variable-volume cell, was conceived and built. The apparatus was tested for the well-known system 1-butanol–CO2. Demixing pressures of the ternary and quaternary mixtures, (1-butanol–water–CO2) and ([C4mim][NTf2]–1-butanol–water–CO2), were determined for a few compositions and temperatures. The demixing pressure is strongly controlled by the water concentration.