The structure and relative energies for the basic conformations of cycloocta-1,5-diyne ( 1), cyclonona-1,5-diyne ( 2), and cyclodeca-1,6-diyne ( 3) have been calculated by the split-valence 6-31G ∗ basis set. The chiral puckered conformer of 1, with a “non-intersecting” twofold axis of symmetry, is calculated to be more stable than the planar, D 2 h , geometry. The plane-symmetrical envelope geometry of 2 is calculated to be a transition-state, separating the unsymmetrical envelope conformers. Ring inversion of the envelope conformation takes place via twist, C 2, transition-state. The chair and boat conformers of 3 have virtually the same energies, and their interconversion take place via the half-chair transition-state.