Compositions of coexisting fluids at elevated conditions can be calculated applying thermodynamics like any other chemical equilibrium. This requires the evaluation of the thermodynamic properties of the phase components at the respective pressure P, temperature T, and the individual phase compositions xi.

Fluids usually have, comparing to solids, a quite distinct PVT-behavior. Furthermore because of their disordered structure and the relatively weak bonding between molecules, fluid species mix much more easily than solid phase components. The evaluation of the thermodynamic properties of fluids requires therefore special treatment and procedures which differ significantly from those used for solids. This includes for fluids a distinct and different standard state than for solids and a special description of the volume V as a function of P, T (or P as a function of V and T) and xi, i.e. equation of state (EOS).