The equilibrium between liquid and vapor ends when the critical temperature is reached. Critical temperature is a characteristic of each substance. The critical temperature of a liquid is the highest temperature at which it can be liquefied, regardless of the pressure. Above this temperature, the substance always is a gas.

This critical temperature can be found on a phase diagram [Local]; the line separating the gaseous phase and the liquid phase ends at this temperature.

Supercritical fluids are increasingly important. In a supercritical fluid, the gas and liquid states have become indistinguishable. Both have the same composition and density.