la-~-Ribofuranosyl-5, 6-dimethylbenzimidazole (a-ribazole) has been prepared by cerous hydroxide catalyzed hydrolysis of cyanocobalamin, purified, and characterized by elemental analysis and'H and I3C NMR and UV-visible spectroscopy. Values of the pKa of N-3- protonated a-ribazole have been determined at several temperatures (ionic strength 1.0 M) and the pK, of N-1-protonated a-ribazole has been estimated to be-7.2 from UV-visible ...