The variation of free-volume parameters (lifetime, intensity, and distribution) after sorption and desorption of CO2 and CH4 gases in the glassy polymer poly[1-(trimethylsilyl)-1-propyne] (PTMSP) and in the rubbery high-density polyethylene (HDPE) and low-density polyethylene (LDPE) were determined by the PAL technique. Size distributions deduced from PAL measurements reveal the presence of large free-volume holes in PTMSP with an average size of 0.725 nm(3) and intensity of 22% in addition to a free-volume hole size of 0.197 nm(3) with an intensity of 11%. In polyethylene free-volume hole sizes of 0.107 and 0.153 nm(3) with intensities of 21% and 25% could be deduced for HDPE and LDPE, respectively.