The waxes appearance in petroleum fluids is mainly due to the decrease of the temperature during the carrying or the exploitation of the fluid. Nevertheless, the influence of the pressure parameter on the waxes formation is far from being negligible. To prevent thoses waxes appearance, predictible thermodynamical models must be developed in order to predict the phase behaviour of such fluids in the temperature and pressure ranges they are submited during their exploitation (0,1 - 100 MPa). In this purpose, two techniques have been developed in order to get experimental data under pressure about the solid phase nature and appearance conditions of synthetic complex systems; those techniques have then been applied on real complex systems. Thanks to those experimental data get under pressure, a totaly predictive model based on a cubic equation of state coupled with some classical mixing rules has been developed. This new model has been built in order to represent the fluid - solid phase equilibrium and the solid phase characterization of some synthetic complex systems, but also some real complex ones, maintained under high pressure.