Down to a cathodic potentials of −1.20 V versus the reversible hydrogen electrode, the structure of IrO₂(110) electrodes supported by TiO₂(110) is found to be stable by in situ synchrotron-based X-ray diffraction. Such high cathodic potentials should lead to reduction to metallic Ir (Pourbaix diagram). From the IrO₂ lattice parameters, determined during cathodic polarization in a H₂SO₄ electrolyte solution (pH 0.4), it is estimated that the unit cell volume increases by 1% due likely to proton incorporation, which is supported by the lack of significant swelling of the IrO₂(110) film derived from X-ray reflectivity experiments. Ex situ X-ray photoelectron spectroscopy suggests that protons are incorporated into the IrO₂(110) lattice below −1.0 V, although Ir remains exclusively in the IV+ oxidation state down to −1.20 V. Obviously, further hydrogenation of the lattice oxygen of IrO₂(110) toward water is suppressed for kinetic reasons and hints at a rate-determining chemical step that cannot be controlled by the electrode potential.