Positron annihilation lifetime spectroscopy was applied to study modification of sub-nanometer voids in one of n-alkanes subjected to high pressure. Without a contact with gaseous atmosphere the sizes of voids behaved as expected - decreased with the rise of pressure. When the pressure was applied with argon penetrating the samples - the voids expanded. After the phase transition the structure damage allowed positronium, trapped in voids, to escape out of a damaged sample. (C) 2004 Elsevier B.V. All rights reserved.