Each of the five C-13 signals of cis-cyclodecene (1) splits into two peaks of equal intensity at lower temperatures, with a free-energy barrier of 6.64 kcal/mol for the process at -139.7 degrees C. The H-1 NMR spectrum of the allylic protons shows decoalescence at -36.4 degrees C, and a free-energy barrier of 10.9 kcal/mol was determined at this temperature. The MM3 strain-energy calculations of Saunders and Jimenez-Vazquez were reproduced to obtain drawings of these conformations, and relative free energies for 1 at -152.7 degrees C were calculated using this program. A single conformation of CI symmetry resembling boat-chair-boat (BCB) cyclodecane was predicted by these calculations and by ab initio calculations at the HF/6-311G* level. C-13 chemical shifts for the preferred conformation of 1 were calculated at the HF/6-311G* level using the GIAO method. cis-Cyclodecene oxide (2) also shows decoalescence in the C-13 spectrum, with Delta G double dagger = 7.4 kcal/mol at -125.4 degrees C. The epoxide has a time-averaged plane of symmetry at room temperature, which no longer exists at -152.6 degrees C. Similarly, the lower barrier of 6.64 kcal/mol for 1 is suggested to be associated with a process which gives the compound a time-averaged plane of symmetry when it is rapid on the NMR time scale.