Any separation technique in which a supercritical fluid is used as the mobile phase. For any fluid, a phase diagram can be constructed to show the regions of temperature and pressure at which gases and liquids, gases and solids, and liquids and solids can coexist. For the gas–liquid equilibrium, there is a certain temperature and pressure, known as the critical temperature (T_c) and pressure (P_c), below which a gas and a liquid can coexist but above which only a single phase (known as a supercritical fluid) can form.