High-resolution crystal structures of lysozyme in the presence of the potential drug (VO)-O-IV-(acetylacetonato)2 under two different experimental conditions have been solved. The crystallographic study reveals the loss of the ligands, the oxidation of V-IV to V-V and the subsequent formation of adducts of the protein with two different polyoxidovanadates: [V4O12](4-), which interacts with lysozyme non-covalently, and the unprecedented [V20O54(NO3)](n-), which is covalenty bound to the side chain of an aspartate residue of symmetry related molecules.