Polyoxometalates are known to be inhibitors of a diverse collection of enzymes, although the specific interactions that lead to this bioactivity are still unclear. Spectroscopic characterization may be an invaluable if indirect tool for remedying this problem, yet this requires clear, cogent assignment of polyoxometalate spectra before the complicating effect of their binding to large biomolecules can be considered. We report the use of FT-IR and resonance Raman spectroscopies alongside density functional theory to describe the vibrational and electronic structures of decavanadate, [V10O28]6–. Our computational model, which reproduced the majority of vibrational features to within 10 cm–1, was used to identify an axial oxo ligand as the most likely position of the acidic proton in the related cluster [HV10O28]5–. As resonance Raman spectroscopy can directly interrogate chromophores embedded in complex systems, this approach may be of general use in answering structural questions about polyoxometalate–enzyme systems.