Positron annihilation lifetime spectroscopy (PALS), differential scanning calorimetry, X-ray diffraction, and polarized light optical microscopy were used to study six low molar mass poly(ethylene oxide) samples with average molar masses ranging from 1 X 10(3) to 10 X 10(3) g mol(-1). Dynamic light scattering was used to determine molar mass and polydispersity rigorously. Polymer samples with 70-95% crystallinity, which is an unusual range in PALS studies, were prepared by molten material quenching. The ortho-positronium pick-off lifetime (tau(3)) and relative fractional free volume (f(v)), determined by the free volume model, correlated well with the average molar mass and crystallinity of the polymers. X-ray diffraction and polarized light optical data support the interpretation of positron annihilation results. PALS parameter, 13, which is associated with high cavity content, remained approximately constant at 20-22% for all samples. The cavities are present as crystallite defects in the spherulitic open texture and the amorphous phase for the low crystallinity sample (e.g., for M-w = 1390) and at the interfaces and in interlamellar spherulite regions of the more crystalline materials. (c) 2007 Wiley Periodicals, Inc.