At critical points, partial derivatives  (:chemical potential of i, :mole fraction of i ) are well known to be equal to zero for mixtures. It drives a curious conclusion that the values of diffusion coefficients will be zero at critical points because the phenomenological  Fick's law is written as . Experimentally, we found that acetone or benzene in supercritical CO₂ around 40 șC shows a minimum near the critical point and a maximum at a little higher pressure. Equations of state also draw similar curves.

   At critical points of  pure substances, instead of  Gibbs energy,  the relation of  is usually used. The region between binodal and spinodal curves is called metastable. Metastable state measurements follow probability. However, spinodal which is limit of stability should be deterministic, because it is defined as a point of . When both temperature and pressure was decreasing in a continuous flow of water, we found transmitted light became darken because of light scattering. We believe that it is a point of spinodal, i.e. as same as the critical point. At a fixed temperature for water, spinodal pressures were higher than the saturation pressures, i.e. binodal curve. Calculated curves with the Peng-Robinson equation of state seem consistent.