Studies were done for providing a simple, rapid and reliable procedure of void volume measurement in packed subcritical fluid chromatography (pSubFC), with CO2-modifier mobile phases containing high modifier amounts. Methods used in RPLC with ODS columns were applied in pSubFC: gravimetric, homologous series linearisation and unretained marker injection. Results lead us to propose the method of marker injection to determine the void volume in pSubFC. Acetonitrile was chosen as the void volume marker among six tested markers. Furthermore, void volume variations vs. the modifier volume (from 5 to 45%) were studied for nine organic modifiers. The void volume variations were related both to adsorption-desorption phenomena between the mobile phase and the stationary phase and to mobile phase density changes. These variations allowed the classification of the modifiers into four groups on the basis of the molecular interactions.