In acidic solutions, the chemical or slow electrochemical reduction of the organic ligand-oxidant 1,10-phenanthroline-5,6-dione consumes two electrons and two protons. However, when the electrochemical reduction is examined on shorter time scales using cyclic or rotating disk voltammetry, fewer than two electrons are consumed in the reduction. The magnitude of the electron deficit depends upon pH and the deficit is eliminated at pH values where the pyridine nitrogen sites on the molecule are not protonated. Complexation of metal cations, e.g., Os(II) or Ni(II), to the 1,10-phenanthroline portion of the molecule also produces an electron deficit in electrochemical reductions of the dione. A proton- or metal ion-induced addition of H2O to the quinone carbonyl groups is shown to be responsible for the observed behavior. Rate and equilibrium constants for the hydration-dehydration equilibrium were evaluated by fitting experimental and calculated parameters in a digital simulation. The possible consequences of the results to studies in which the dione is used to oxidize amines and other organic reductants are pointed out.