The ten-membered lactone, oxacyclodecan-2-one (1), was found by low-temperature H-1 and C-13 NMR spectroscopy to have two conformations with populations of 95.2 and 4.8% at -133.8 degrees C. Free-energy barriers of 9.09 +/- 0.15 and 8.26 +/- 0.15 kcal/mol for interconversion of conformations at -110.7 degrees C were calculated from C-13 NMR spectra, and a free-energy barrier of 9.1 +/- 0.2 kcal/mol at -70.8 degrees C was determined for another process, observed only in the H-1 NMR spectra. The barrier for the process observed in the proton NMR spectra was suggested to be a lower limit for the barrier to rotation of the lactone group through the ring. Oxacycloundecan-2-one (2) has two conformations at -139.3 degrees C, with populations of 64.3 and 35.7% at this temperature and free-energy barriers to interconversion of 7.83 +/- 0.15 and 7.67 +/- 0.15 kcal/mol at -109.1 degrees C. The NMR results for both compounds are discussed in terms of the conformations predicted by Allinger＇s MM3 program. Ab initio calculations of electronic energies for three conformations of the 10-membered lactone were also carried out at the HF/6-311+G* and MP2/6-31G* levels.