The effect of oxygen deficiency in Cu-based spinels Li[Cu0.5Mn1.5](oct)O4-delta was examined using first-principles calculation. Two main results were obtained upon oxygen removal: (i) a progressive unit cell expansion owing to the presence of both more reduced transition metal cation and oxygen vacancies, (ii) a progressive decrease in the average lithium deinsertion voltage as the oxygen deficiency increases. Computational results indicate that the oxygen deficiency gives rise to a partial reduction of Mn4+ to Mn3+, whereas upon lithium deinsertion, this reaction is reversed together with the oxidation of Cu2+ to diamagnetic Cu3+. The electrochemical tests in lithium cells of two spinels with delta values 0.04 and 0.1 were consistent with these calculations. Computational data indicate that the oxidation of Cu+2 to Cu+3 would be favored in highly oxygen deficient spinels; this contradicts the experimental results that show a decrease of the specific capacity in the high-voltage range with the oxygen deficiency. However, in the prepared spinels, there is an important fraction of Cu ions occupying tetrahedral sites. Therefore, computational investigation points to the electrochemical inactivity of tetrahedral Cu rather than to the oxygen content as the origin of the poor charge capacity observed in the high-voltage range. (c) 2006 The Electrochemical Society.