A simple device for the preparation of a ＇＇modified＇＇ supercritical fluid consisting of carbon dioxide with a constant concentration of ＇＇modifier＇＇ is described. The system is based on the thermodynamic equilibrium obtained in a ＇＇saturator＇＇ which permits the contact of carbon dioxide with the modifier at pressure and temperature conditions determining the mobile phase composition with high reproducibility and reliability at low cost.