Abstract Polarized IR spectra of H12 345 2-methylimidazole and of its H1D2 345, D1H2 345 and D12 345 deuterium derivative crystals are reported and interpreted within the limits of the “ strong- coupling” theory. The spectra interpretation facilitated the recognition of the H/D isotopic “ self-organization” phenomenon, which depends on a non-random distribution of protons and deuterons in the lattices of isotopically diluted crystal samples. The H/D isotopic “ self- organization” mechanism engaged all four hydrogen bonds from each unit cell. These effects basically resulted from the dynamical co- operative interactions involving adjacent hydrogen bonds in each hydrogen bond chain. A weaker exciton coupling involved the closely spaced hydrogen bonds; each belonging to a different chain of associated 2-methylimidazole molecules. The high intensity of the narrow band at ca. 1880 cm −1 was interpreted as the result of coupling between the γ N–H⋯N proton bending “ out of plane” vibration overtone and the ν N–H proton stretching vibration.