The interfacial chemical reactivity of Jahn-Teller-active transition-metal oxides remains an enigmatic area, often leading to uncontrollable phase transformations in the oxide-based technological applications. In particular, the higher tendency of unwanted transition-metal-ion dissolution and side-reactions in Jahn-Teller-active oxides is accompanied by performance degradation in many electrochemical systems, for example, lithium-ion batteries. We show here that the fundamental electronic structure instability that leads to Jahn-Teller (lattice) distortion in an octahedral ligand field is the active chemical driving force for the spontaneous disproportionation, phase transformation, and metal-ion dissolution in transition-metal oxides upon exposure to protons. On the basis of electronic structure analyses and 18O isotope labeling, we present a mechanism comprising a coupled acid-base/redox reaction that leads to a proton-induced disproportionation of Jahn-Teller-active transition-metal ions, as exemplified by the broad classes of respective oxides.