The combined effect of pressure and temperature will be analyzed in two types of fluids : metal molten salts and ionic electrolytes. In the metal molten salts our interest will be focused on the evolution of the electron-localization and their possible interaction with the ionic clustering . Ionic electrolytes will be discussed in the light of recent experiments and models, in particular the observed reduction of the ion hydration shells and the predicted appearance of ion paring near the critical conditions.
The study by means of EXAFS of the non-metal to metal transition of selenium near the critical point will serve to illustrate the experimental difficulties encountered in this type of studies and the extreme precautions that must be taken in the data analysis. In particular we will underline the strong non-harmonic analysis that is needed to avoid misinterpretations and that leads to very interesting physical results. The relationship between structural/transport changes and the 'crossover' of the critical conditions will be examined.