This paper is devoted to the description of a method for modelling heavy cuts, using either a T.B.P. distillation or an advanced chromatography analysis method. Two sorts of reservoir fluids are defined depending on the advancement of the geochemical catagenetic reactions occurring during the history of the petroleum deposit. Given the existence of differences due to the geochemical history of fluids, two different thermodynamic characterizations may be computed from any given analytical information, One is generally more suitable for use with condensate gases and the other is more appropriate for calculating the physical properties of crude oils, The method presented here involves tuning one or two parameters to one of several saturation pressures of the reservoir fluid. This approach was tested on a data base including more than 50 reservoir fluids. In most cases, the PVT properties predicted using the above method were highly satisfactory.