Multinuclear solid-state nuclear magnetic resonance methods (Co-59, C-13, N-15, and P-31 NMR) were applied at natural abundance to the structural and dynamic analysis of cyanocobalamin ( vitamin B-12) polymorphs. These studies involved recrystallizing a series of samples under different conditions and from various solvents, and subsequently recording their powder NMR spectra at different temperature. Two pofq morphs could be identified in these studies, in correspondence with the two structures described by Hodgkin and co-workers in their seminal vitamin B-12 crystallographic analyses, Most informative about the molecular differences characterizing these two forms were the C-13 NMR data, which showed sharp and well-resolved resonances indicative of high sample crystallinity. Diagnostic differences between these two forms could be observed in the chemical shifts of particular resonances, many of which could be assigned to their molecular sites on the basis of solution-state Literature and of solid-state spectral editing procedures. These shifts indicated a conformational variability that involved a few specific sites in the corrinoid ring and which was not entirely evident from differences among the X-ray structures previously reported for the forms. Further evidence about the conformational flexibility of these sites in the solid is furnished by C-13 spectral shifts observed upon changing temperatures. The relevance of these observations within the context of the extensive structure/ activity relation studies that have been carried out on this class of systems is briefly addressed.