A method of monitoring a CO2 geological storage site by locating the CO2 plume from 4D seismic data is disclosed. A stratigraphic inversion of the seismic data is performed in order to obtain the P and S impedances, before and after CO2 injection. A density variation cube and an incompressibility modulus variation cube are constructed from the seismic impedances. The free CO2 plume is located within the subsoil by identifying, within the cubes cells where the density variation is negative and where the incompressibility modulus variation is negative, which are of an absolute value greater than a given positive threshold.