The slow-exchange C-13 spectrum of trans-cyclodecene at -154.9 degrees C shows eight peaks for the olefinic carbons, and these are interpreted in terms of five conformations. Three of the conformations are of C-1 symmetry, and two are of C-2 symmetry. Further evidence for the number of conformations and their symmetries came from a proton NMR spectrum of the olefinic hydrogens taken at -154.9 degrees C with decoupling the allylic hydrogens. Populations ranged from 3.0% to 37.6%, with the least-populated conformation having a free energy of 0.59 kcal/mol, relative to the most stable conformer. The conformations studied by Saunders and Jimenez-Vazquez using Allinger’s MM3 force field are described, and the calculated strain energies and populations are discussed. Energies for six conformations were also obtained from ab initio calculations at the HF/6-311G* level.