Lignin is the second most abundant natural organic polymer on earth, being the first one among those composed of aromatic units. Lignin can be a renewable source of valuable phenolic chemicals, if converted to smaller molecular units, thus representing a potential bio-based alternative to several petrochemical processes. New processes for producing high-purity kraft lignin from wood have recently become commercial and, thus, in the years to come there will be a large amount of this bio-material available for future biorefineries. Among the possible processes for lignin depolymerisation and cracking, the conversion in near-critical or supercritical water is among the most promising. A version of this reactive process is currently under investigation in a laboratory plant at Chalmers University of Technology (Chalmers), but several fundamental aspects of the process are still to be understood. This research proposal aims at integrating the activities currently carried out at Chalmers, focusing on fundamental aspects which are relevant for the process under investigation. More specifically, the proposed project aims at investigating the phase equilibria of the reacting system, composed of lignin and its derivatives, water at near-critical or supercritical conditions, co-solvents, and salts. The phase equilibrium analysis aims at providing information about the number and the nature of the phases constituting the system, depending on the operating parameters (e.g., pressure, temperature, concentration of salts). To reach this goal, the planned activities comprise the realization of a visual apparatus for measuring phase equilibrium at high-pressure and high-temperature, the measurements on samples taken from the reactor currently in use at Chalmers and on representative synthetic systems, and the development of thermodynamic modelling. The project is expected to find original results on this specific system. The project spans over a period of four years.