The effect of oxygen non-stoichiometry on the electrochemical performance of lithium manganese oxide spinels is investigated. Spinels with different oxygen contents are obtained from citrate gel synthesis followed by calcination in air in the temperature range 200–850°C. Spinels that were formed at low temperatures are found suitable for 3V applications, whereas those formed at high temperatures perform better at 4V. Both type of spinels cycle well in their respective potential regimes.