Processes using water at sub- and supercritical conditions (T > 300 °C) are used in industrial wastewater treatment, chemical recycling of wastes, biomass conversion and also in material synthesis, because of their rapidity and high efficiency. However, the salts injected or generated during the processes can lead to complex multiphasic water-salts systems with a behavior far to be understood in all the cases. Drastic changes of physics and chemistry of the systems are expected from water intrinsic properties; however, the formation of new phases, sometimes unexpected, near the critical point of water (374 °C; 22.1 MPa) also has a direct effect on the properties of the system. It is thus important to be conscious of the existence of these new phases to have the best control as possible on the physico-chemistry occurring inside the reactor. The aim of this review paper is to give an overview of these poorly-known new phases and to show that they are more encountered in SCW than it is thought.