Density functional theory and ab initio MP2 6-31G* calculations were carried out to investigate the structures and vibrational spectra of 3,4- and 2,3-pyridyne. It is found that the structure of 3,4-pyridyne is consistent with a formal CC triple bond moiety, but the structure of 2,3-pyridyne is more properly described as having a C double bond C double bond N unit. On the basis of the calculated results, detailed assignments of the observed IR bands of 3,4-pyridyne are proposed. The calculations predict the most prominent IR feature of 2,3-pyridyne is a very strong band around 1826 cm(-1), suggesting the search for direct experimental evidence of 2,3-pyridyne should pay attention to this spectral region.