Microwave spectra of NCCCH -NH 3 , CH 3 CCH-NH 3 , and NCCCH -OH 2 have been recorded using a pulsed-nozzle Fourier-transform microwave spectrometer. The complexes NCCCH -NH 3 and CH 3 CCH-NH 3 are found to have symmetric-top structures with the acetylenic proton hydrogen bonded to the nitrogen of the NH 3 . The data for CH 3 CCH-NH 3 are further consistent with free or nearly free internal rotation of the methyl top against the ammonia top. For NCCCH -OH 2 , the acetylenic proton is hydrogen bonded to the oxygen of the water. The complex has a dynamical C 2v structure, as evidenced by the presence of two nuclear-spin modifications of the complex. The hydrogen bond lengths and hydrogen-bond stretching force constants are 2.212 A ˚and 10.8 N /m, 2.322 A ˚and 6.0 N /m, and 2.125 A ˚and 9.6 N /m for NCCCH -NH 3 , CH 3 CCH-NH 3 , and NCCCH -OH 2 , respectively. For the cyanoacetylene complexes, these bond lengths and force constants lie between the values for the related hydrogen cyanide and acetylene complexes of NH 3 and H 2 O. The NH 3 bending and weak-bond stretching force constants for CH 3 CCH -NH 3 are less than those found in NCCCH -NH 3 , NCH-NH 3 , and HCCH -NH 3 , suggesting that the hydrogen bonding interaction is particularly weak in CH 3 CCH-NH 3 . The weakness of this hydrogen bond is partially a consequence of the orientation of the monomer electric dipole moments in the complex. In CH 3 CCH-NH 3 the antialigned monomer dipole moments lead to a repulsive dipole -dipole interaction energy, while in NCH -NH 3 and NCCCH -NH 3 the aligned dipoles give an attraction interaction.