Positron lifetime measurements were carried out in a series of poly(ethylene oxide)-PEO-of different average molecular weights ((M) over bar(w)): 1000, 1500, 6000, 10,000, 300,000, and 4 M. The mean radius ((R) over bar) and the mean free volume size (Vf) values were determined using a semiempirical equation that correlates the orthopositronium (o-Ps) lifetime (tau(3)) and size of holes existing in the amorphous phases. The hole mean radius values determined at room temperature from lifetime spectra were found to be between 2.68 and 2.97 Angstrom, and the hole volumes between 80 and 110 Angstrom(3). Free volume size evolution was studied with temperature variation until the melting temperature of the PEO samples. The degree of crystallinity and the melting temperatures were determined by Differential Scanning Calorimetry.