Chelating molecules in radiopharmaceutical design play an important role as anchors for radionuclide metals and for targeting vector macromolecules. In the case of 89 Zr, the bacteria-produced siderophore desferrioxamine B (DFO) is most used for radiolabeling. DFO is an open-chain, hexadentate chelate molecule having three hydroxamate groups as radiometal binding moieties and a terminal primary amine that allows conjugation with vector biomolecules. In 1964, Baroncelli et al. found a high affinity of Zr 4+ ions towards hydroxamic acid groups and in 1992, Meijs et al. reported successful radiolabeling of DFO with Zr and good in vitro stability of the resulting complex. Since then, many other DFO analogues and other chelators, including acyclic and cyclic polyazacarboxylates, have been studied in search of complexes which are more stable in vivo and avoid unspecific uptake of 89 Zr in, for example, the bones. More recently over the last decade, the speciation and thermodynamic stability of Zr 4+ -DFO complexes in solution has been investigated through potentiometric, spectrophotometric and mass spectrometry measurements. These studies have pointed to the possibility of the formation of non-mononuclear complexes involving DFO and Zr 4+ .