The effect of mechanical milling on free volume was studied by means of positron annihilation lifetime spectroscopy (PALS) in polystyrene (PS) as a typical brittle polymer and in polycarbonate (PC) as a tough representative. Long-time milling increases the free volume, while a decrease is observed for short milling times. The changes are mostly irreversible in PS. The irreversible fraction is much smaller for PC. Gel permeation chromatography (GPC) measurements show a decrease of the molecular weight, which is much more pronounced in PS. The milling-induced irreversible changes in free volume are attributed to chain-end defects resulting from chain scission. In PC, other deformation-induced defects that anneal upon heat treatment above the glass transition temperature dominate. (C) 2004 Society of Plastics Engineers.