Abstract The 1H and 13C nmr spectra of Co(NH 3) 5ImH 3+ and the 1H nmr spectra of αCotrien(ImH) 2 3+ and βCotrien(ImH) 2 3+ are reported. The p K a values determined from the dependence of the chemical shift on pH are 10.0, 9.6, and 10.1, respectively. The range of the chemical shift between the acid and base forms is unusually small in the 1H nmr, 0.5–0.7 ppm for the C-2 H and about 0.25 ppm for the C-4 H and C-5 H. In the 13C nmr, C-2 and C-4 have large shifts to low field and C-5 a small shift to high field on deprotonation. The C-2 proton is not exchanged with solvent 2H under acidic or basic conditions, in marked contrast to the corresponding proton in both imidazole and 1-methylimidazole. These spectroscopic and chemical properties should be useful for the direct identification of metal-ion coordinated histidines in proteins.