Typically, supercritical fluids are completely miscible with each other, so that a binary mixture forms a single gaseous phase if the critical point of the mixture is exceeded. However, exceptions are known in systems where one component is much more volatile than the other, which in some cases form two immiscible gas phases at high pressure and temperatures above the component critical points. This behavior has been found for example in the systems N-NH, NH-CH, SO-N and n-butane-HO.
The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,
where χ denotes the mole fraction of component i.
For greater accuracy, the critical point can be calculated using equations of state, such as the Peng–Robinson, or group-contribution methods. Other properties, such as density, can also be calculated using equations of state.