For the first time, the effect of the oxygen anion substitution on the reversible electrochemical activity of metal fluorides was investigated through the use of bismuth oxyfluorides. We demonstrate that a reversible conversion occurs in BiOxF3-2x ((x = 0,0.5,1,1.5))/C nanocomposites with the formation of Bi-0, Li2O, and/or LiF during lithiation and the reformation of BiOxF3-2x during subsequent delithiation as opposed to a two-phase mixture of Bi2O3 + BiF3. We also show evidence that in BiOF and BiO0.5F2 the fluoride component reacts first at higher voltage during lithiation to form Bi-0, LiF, and Bi2O3. The newly formed Bi2O3 is then subsequently reduced into Li2O and Bi-0 on a second plateau at lower voltage. During delithiation the process is reversed, Li2O reacting first at low voltage followed by LiF. Although cycling is presently poor, our results show that a relatively low oxygen content can significantly improve the electrochemical activity of the metal fluoride, suggesting that optimized metal oxyfluorides could be an attractive alternative to metal fluorides for applications as positive electrode materials in lithium batteries. (c) 2005 The Electrochemical Society.