Experiments are described to elucidate the structure and solvation of aquocobalamin (vitamin B-12a, If) in the crystal and in aqueous solution. Aquocobalamin (1(+)) is the B-12 derivative in which a water molecule replaces the axially coordinating organic substituent (methyl or 5’-desoxyadenosyl) at the beta-side of the cobalt of the B-12 coenzymes. (1) A single-crystal structure analysis of aquocobalamin perchlorate (1(+)ClO(4)(-)), using synchrotron radiation in combination with an imaging plate detector, yielded the most accurate structural data ever determined for a B-12 molecule. 1(+)ClO(4)(-) crystallizes in the orthorhombic space group P2(l)2(l)2(l), a = 15.042(11) Angstrom, b = 23.715(14) Angstrom, c = 25.104(12) Angstrom, with four 1(+)ClO(4)(-) moieties plus about 100 solvent water molecules per unit cell; 22867 independent and 20942 significant intensity data were recorded to a nominal resolution of 0.8 Angstrom, and refinement against F-2 quantities led to a conventional R-value of 0.050 for all 22867 observations and to a structural model with an average ESD for all carbon-carbon bonds of 0.003 Angstrom.