Selectively 15N-labeled tetrazolo[1,5-b][1,2,4]triazines and tetrazolo[1,5-a]pyrimidines bearing one, two, or three 15N labels were synthesized. The synthesized compounds were studied by 1H, 13C, and 15N NMR spectroscopy in DMSO and TFA solutions, where the azide–tetrazole equilibrium can lead to the formation of two tetrazole (T, T′) isomers and one azide (A) isomer for each compound. Incorporation of the 15N-label(s) leads to the appearance of 15N–15N coupling constants (JNN), which can be easily measured via simple 1D 15N NMR spectra, even at natural abundance between labeled and unlabeled 15N atoms. The chemical shifts for the 15N nuclei in the azole moiety are very sensitive to the ring opening and azide formation, thus providing information about the azido–tetrazole equilibrium. At the same time, the 1–2JNN couplings between 15N-labeled atoms in the azole and azine fragments unambiguously determine the fusion type between tetrazole and azine rings in the cyclic isomers T and T′. Thus, combined analysis of 15N chemical shifts and JNN values in selectively isotope-enriched compounds provides an effective diagnostic tool for direct structural determination of tetrazole isomers and azide form in solution. This method was found to be the most simple and efficient way to study the azido–tetrazole equilibrium.