The electronic and vibrational spectra of the 1: 1 clusters of 2-aminopyridine (2AP) with ammonia have been measured using resonant two-photon ionization (R2PI) and IR/R2PI vibrational spectroscopy. Density functional theory calculations were performed to identify structures and assign the vibrational spectra in the NH stretch region. The two lowest-energy isomers have been identified. In isomer 1, which is the global minimum energy structure, ammonia forms a strong H bond with the amino group and a weak one with the aromatic nitrogen. The vibrations of the groups involved in this H bonding exhibit red shifts of I 10 cm(-1) for the donating amino N-H and of 69 cm(-1) for the donating N-H group of ammonia. The degeneracy of the two asymmetrical vibrations of ammonia is removed in the cluster, with the corresponding bands at 3438 and 3406 cm(-1). In isomer 11 ammonia forms an H bond to the N-H group which points away from the aromatic nitrogen. This bond is weaker than the corresponding one in isomer I. The stretching vibration of the H bonded N-H in the amino group exhibits a red shift of 66 cm-1, whereas the vibrations of ammonia are very similar to those of isolated ammonia. This gives evidence that ammonia is a pure H bond acceptor. The exclusiveness of these structural isomers is due to the dominant role of the proton affinity of ammonia in the H bond. The spectroscopic results are well supported by the calculations performed at the density functional level of theory. In a similar spectroscopic stud), of 2AP with one or two water molecules, water forms in-plane bridges between the ring nitrogen and the amino group. However, the H bond with the ring nitrogen is dominant and only one lowest-energy isomer could be found for each cluster size