The oxidized and hydroquinone forms of synthetic 8α-N-imidazolylriboflavin have been investigated by proton nuclear magnetic resonance spectroscopy at 360 MHz. Proton resonances due to the imidazole ring, isoalloxazine ring, and ribityl side chain have been assigned on the basis of two-dimensional 1H-1H correlated spectra (COSY), selective decoupling, and nuclear Overhauser effect difference spectra and by comparison of computer-simulated with experimental spectra. The effect of pH on the imidazolyl resonances shows a pKa for the unsubstituted imidazole nitrogen of 6.0 ±0.1 for the oxidized form and a value of 7.0 ±0.1 for the reduced form, in good agreement with the values obtained from oxidation-reduction potential data in a previous paper [Williamson, G., & Edmondson, D. E. (1985) Biochemistry 24, 7790-7797]. Slow exchange of the flavin 8α-methylene and imidazolyl C(2) protons was observed at pH 6.1 but not at pH values below 4.0 for the oxidized form of the flavin. The reduced form, but not the oxidized form, of the flavin exhibits geminal coupling of the 8α-methylene protons and of the C(l') methylene protons of the ribityl side chain. The magnetic nonequivalence of the protons of these two methylene groups is suggested to result from intermolecular association of the reduced flavin in aqueous solutions at the concentrations required for the spectral experiments.