The electrochemical intercalation of oxygen from an aqueous KOH electrolyte into La2NiO4+delta has been investigated by a potential step method at potentials below oxygen evolution. The first reduction cycle occurs at nearly constant potential for the whole composition range and is characteristic of nucleation and growth behavior. On the first oxidation and subsequent reduction-oxidation cycles, the results are similar to those obtained previously for polycrystalline electrodes and show the presence of a two-phase region for 0 less than or equal to delta less than or equal to 0.06 and a single phase with the composition 0.06 less than or equal to delta less than or equal to 0.13. The P4(2)/ncm phase with delta similar to 0.02 and a narrow range of stoichiometry, is apparent in d delta/dV versus delta data and in the behavior of the current decay curves. Examination of the crystals at various stages of cycling show the formation of extensive cracks that apparently increase in density with cycling. Measurements of the transient currents after potential steps have been used to estimate the diffusion coefficient of oxygen in La2NiO4+delta in the single phase region. Data were obtained for both polycrystalline and single crystal electrodes.