We sought to use electrospray ionization high resolution mass spectrometry (ESI-HR-MS) to obtain the mass of the clusters. Unfortunately, our carboxylate capped clusters show limited solubility in typical ESI-HR-MS solvents such as methanol and acetonitrile. Zr12acetate clusters dissolve in methanol but no clusters were detected in ESI-HR-MS and NMR analysis provides evidence for cluster degradation in methanol, see Figure -17. The limited solubility of Zr12-acetate did not allow for any further HR-MS analysis. On the other end, the Zr12-oleate clusters were too non-polar. The other clusters could be dissolved in tetrahydrofuran (THF) and were analyzed with ESI-HR-MS after adding acetonitrile as co-solvent (full spectra and assignments in figures S18-25). The Zr6 clusters give reasonably clean spectra where the molecular ion can be recognized as Zr 6 O 4 (OH) 3 (L) + 12 (the cluster minus an OH -group). The example of Zr6-methylbutaonate is shown in Figure . The Z12 clusters appear less stable and the ion [Zr 6 O 4 (OH) 7 (L) 9 ] 2 H + was observed in most cases (see Figure ). This ion is the proton adduct of the Zr12 dimer after substituting six carboxylates for six hydroxides. The Zr12 cluster features indeed 6 chelating ligands, which are most labile according to literature. It is thus plausible that the such an exchange takes place, further catalyzed by a coordinating solvent like THF. Interestingly, more fragmentation was seen for longer ligands and the monomeric form is even dominant for Zr12-decanoate and Zr12-dodecanoate. Despite the limitations, MS thus confirms the successful formation of oxo cluster with long carboxylate ligands. It is however not the correct technique to distinguish the dimer from the monomer structure.