AbstractQuaternary13C dipole–dipole spin–lattice relaxation times are used to analyse prototropic equilibria of 2‐acetylbenzimidazole (1) and 4‐azabenzimidazole in DMSO‐d6solutions. For the first compound, fast exchange between the two chelated structures prevails for13C relaxation times, whereas proton and13C chemical shifts characterize a slow exchange. For 4‐azabenzimidazole,13C and1H spectra do not exhibit absorptions from structures in slow exchange. Analysis of theT1values of carbons 8 and 9 before and after isotopic exchange of the labile amino hydrogen clearly show that the major component of the equilibrium is the