Although butane exists in staggered anti and gauche conformations, when the ethyl groups are separated by a CC triple bond (3-hexyne), the stable conformation changes to eclipsed,(1) C-2v. Using rotational microwave spectroscopy, we have studied another example, 3-heptyne, the CC elongated analogue of pentane. The most stable conformer of pentane has anti-anti (AA) conformations about the central CC bonds (C-2v) and the next most stable has a gauche dihedral angle (GA, C-1). This microwave study determines that the extended analogue of the AA form is not staggered about the CC axis but eclipsed (C-s). Also, the elongated analogue of the GA conformer is also not staggered but nearly eclipsed. The conformations of low-polarity substituted acetylenes is determined by dispersion attractions between the end groups. A microwave study of the AA and GA conformers of pentane is also reported.