The structure of hexadeca-mu-acetato-tetraaquadodeca-mu3 oxo-dodecamanganese bis(acetic acid) tetrahydrate, [Mn12O12 (CH3COO)16(H2O)4].2CH3COOH.4H2O, known as Mn-12- acetate, has been determined at 83(2) K by X-ray diffraction methods. The fourfold (S-4) molecular symmetry is disrupted by a strong hydrogen-bonding interaction with the disordered acetic acid molecule of solvation, which displaces one of the acetate ligands in the cluster. Up to six Mn-12 isomers are potentially present in the crystal lattice, which differ in the number and arrangement of hydrogen-bonded acetic acid molecules. These results considerably improve the structural information available on this molecular nanomagnet, which was first synthesized and characterized by Lis [Acta Cryst. (1980), B36, 2042-2046].