The behavior of the variable-composition spinel Li1 + xMn2 − xO4 is examined in repeated cycles consisting of lithiation in 0.2 M LiOH and delithiation in 0.3 M HNO3. For 0 < x < 0.33, delithiation is accompanied by the redox reaction 2Mn3+ → Mn4+ + Mn2+ and Li+ ⇄ H+ ion exchange. The spinel undergoes partial conversion into λ-□MnO2. Vacancies (□) build up at the 8a sites of the spinel structure. Mn2+ ions pass into the solution, and, accordingly, the spinel dissolves. Lithiation is accompanied by the redox reaction 4Mn4+ → 3Mn3+ + Mn7+ and ion exchange, and the proportion of vacancies □ at the 8a sites of the spinel structure decreases. The spinel undergoes partial dissolution because of Mn2+ and MnO−4 ions passing into the solution. The Li+ selectivity of the spinel is the property of the crystallite core. The crystallite surface is capable of sorbing Na+ ions.