The conformations of azacyclodeca-3,8-diyne (4a) and its N-methyl (4b) and N-isopropyl (4c) derivatives as well as 1,6-diazacyclodeca-3,8-diyne (5a) and its 1,6-dimethyl (5b), 1,6-diisopropyl (5c), 1,6-di-tert-butyl (5d), and 1,6-ditolyl (5e) derivatives have been investigated by single-crystal X-ray diffraction, in solution by NMR and by theoretical methods. In the solid state, 4a,c and 5a-e adopt a chair conformation with the substituents in the bis-axial positions. In solution 4 and 5 show Delta G(double dagger) = 10.4 and 11.0 kcal/mol, respectively, for ring inversion. Trapping of 5e with HCl indicates the presence of bis-axial, axial, and equatorial conformations in solution. The equilibrium between boat and chair conformations in 4 and 5 is ascribed to much reduced torsional strain between the propargylic hydrogens. The preference of the axial orientation of the substituents on the nitrogen atom(s) in 4 and 5 is interpreted in terms of the vicinal interactions of both the NR bond and the lone pair with the adjacent CH and CC bonds, synaxial 1,3-interactions of both the NR bond and the lone pair, and secondary interactions of CH/CC bonds of the alkyl substituent with the endocyclic CH2 groups as well as with triple bonds (back and front strain).