5-Tropolonediazonium salt 1 is a well-known intermediate for the preparation of 5-substituted tropolone derivatives, but 1,2,5-tropoquinone-5-diazide 2, which is expected to be formed by deprotonation of 1, has not been reported. We synthesized 2, and the structures of 1 and 2 were investigated and compared. NMR and UV spectral data indicated that 1 is easily deprotonated in water, methanol, DMSO, and DMF and exists in the form of 2 in these solvents (but not in acetone or acetonitrile) because of its strong acidity (estimated pKa −2.07). Thus, the acid–base equilibrium shows strong solvent-dependence. Compound 2 may be synthetically available as a carbene precursor.