We have prepared the first known lithium manganese oxynitride, Li7.9MnN3.2O1.6, and characterized both its structure and electrochemistry. Powder diffraction experiments show that it adopts a cubic antifluorite lattice, while electron microscopy studies reveal the existence of an incommensurate modulated superstructure due to partial Li/Mn and/or O/N ordering. This is consistent with the Li-6 magic angle spinning-nuclear magnetic resonance spectra, which show only a limited number of Li local environments. Li7.9MnN3.2O1.6 is electrochemically active when tested as an electrode in a lithium battery and can be reversibly oxidized up to 1.9 V. Additional reversible capacity is obtained on reduction down to 0 V due to the insertion of Li onto the existing lithium-ion vacancies. In situ X-ray diffraction experiments indicate that the redox mechanism is single-phase and involves the formation of a solid solution Li7.9 +/-chiMnN3.2O1.6. This compound exhibits an overall specific capacity of 310 mAh/g, with only a 4% loss after the 40th cycle. The higher capacity retention of Li7.9MnN3.2O1.6 when compared to that of Li7MnN4 is due to both the easier diffusion of the lithium within the structure caused by the additional cationic vacancies, and the fact that its lithium exchange mechanism goes through the formation of a solid solution, with no biphasic regions. (c) 2005 The Electrochemical Society.