Abstract: A containment cell with two chambers (7) in a thermostat-controlled enclosure (3) is used to measure separately, at determined pressure levels, the volumes occupied separately by fluid phases, prior to measuring at the same pressures the volume occupied by the mixture thereof, so as to deduce therefrom the excess volume (v.sup.E) and the density of the mixture. By minimizing an error function (F) that takes account of volume quantities, such as the excess volume (v.sup.E) or the density of the mixture, and of phase equilibrium data of the mixture such as the saturation pressures (Psat), these data being determined from said cell, it is possible to determine with precision the coefficients of the equation of state relating the thermodynamic parameters of the mixture (P, V, T) and thereby to fine down for example reservoir models. The method can be applied for study of petroleum fluids for example.