Crystalline cyclic poly(oxyethylene)s of number-average molar mass 1000, 1500, 2000 and 3000 g mol(-1) were studied by high-frequency and low-frequency laser-Raman spectroscopy, wide-angle (WAXS) and small-angle (SAXS) X-ray scattering, and differential scanning calorimetry (d.s.c.). Comparison was made with the properties of corresponding linear poly(oxyethylene) dimethyl ethers. The crystal structures of the linear and cyclic polymers were essentially the same as that of high-molar-mass poly(oxyethylene). The cyclic polymers crystallized in the twice-folded conformation and the linear polymers in the extended conformation, as confirmed by their lamellar spacings (SAXS) and the frequencies of their single-node longitudinal acoustic modes (LAM-1). Compared al identical lamellar spacing, the cyclic polymers had identical enthalpies of fusion (per mol of chain units) as the linear polymers but higher melting temperatures, consistent with lower entropies of fusion (per mol of chain units) for the cyclic polymers.