The electrochemical cycling of a hydrated, iron-substituted nickel hydroxide (20 atomic percent of iron substituted for nickel) in a Ni//Cd cell was studied insitu by Fe-57 Mossbauer spectroscopy. The variations of the iron oxidation state (and spin configuration) were followed throughout the discharge and correlated to the changes of the overall distribution of cationic oxidation states, as well as to the structural modifications. The charge starting from an alpha-phase leads to a pure gamma oxyhydroxide containing Ni-III, Ni-IV, Fe-IV, and average valence Fe-<3.5+> ions. During the discharge, the evolution of the various cationic populations deduced from the Mossbauer data provides evidence of a gamma-type solid solution domain, in which the Ni-IV ions are reduced to the trivalent state rather than the iron ions, followed by gamma + alpha biphased domain.