The complex Ru(II)(NH(3))(2)(o-benzoquinonediimine)Cl(2) undergoes a reversible apparent acid/base reaction, although it has no obvious basic lone pairs. The reaction is a proton-assisted disproportionation yielding an oxidant ([Ru(III)(NH(3))(2)(o-benzoquinonediimine)Cl(2)](+)) and a reductant ([Ru(III)(NH(3))(2)(o-phenylenediamine)Cl(2)](+)). These species were characterized by electrochemistry, ultraviolet-visible light (UV-vis), vibrational (infrared (IR) and Raman), mass and electron paramagnetic resonance (EPR) spectroscopy, and X-ray structural analysis. The reaction is shown to be downhill from an isodesmic calculation. Three different isosbestic interconversions of the parent and product species are demonstrated. The electronic structures of these species were analyzed, and their optical spectra assigned, using density functional theory (DFT) and time-dependent DFT. This disproportionation of a noninnocent ligand complex may be relevant to the application of noninnocent ligands in organometallic catalysis and in the biological milieu.