Adenosylcobinamide (AdoCbi(+)BF(4)(-)), the base-free form of adenosylcobalamin (AdoCbl or coenzyme B-12), has been studied with a series of 14 exogenous alpha-axial bases, Specifically, equilibrium association constants, K-assoc, as a function of temperature were measured, and thus their associated Delta H and Delta S were obtained. Bases studied include the following : (i) exogenous 1,5,6-trimethylbenzimidazole [analogous to adenosylcobalamin’s intramolecularly appended 5,6-dimethylbenzimidazole base]; (ii) sterically encumbered phosphine bases (none of which showed detectable binding in dramatic contrast to studies of, for example, cobaloxime B-12 "models"); and (iii) electronically increasingly donating, but isosteric, 4-substituted pyridine axial bases. The general trends from the present K-assoc studies are 2-fold : the more electron donating the base, the greater the K-assoc, and bulky bases bind weakly if at all. This paper also contains a tabular summary of the existing, non-Ado RCbi(+) axial-base K-assoc literature plus the relatively few associated Delta H and Delta S values that are available. Selected B-12 model axial-base K-assoc literature is also summarized as Supporting Information. In addition, the Discussion contains a critical analysis of the prior, B-12 enzymic biochemical literature relevant to the role of AdoCbl’s appended 5,6-dimethylbenzimidazole axial base.