The effect of the substitution of a small amount of Al3+ or F- on the well- known irreversible loss of oxygen during the first charge from the layered Li[Li0.17Mn0.58Ni0.25]O-2, which is a solid solution between Li[Li1/3Mn2/3]O-2 and Li[Mn0.5Ni0.5]O-2, has been investigated. The oxygen loss from both Li[Li0.17-xAlxMn0.58Ni0.25]O-2 (0 <= x <= 0.1) and Li[Li0.17Mn0.58Ni0.25]O2-yFy (0 <= y <= 0.1) decreases sensitively with increasing Al or F content, which results in a drastic decrease in the reversible capacity in the subsequent cycles from similar to 250 to 90 mAh/g for x = 0.1 and 120 mAh/g for y = 0.1. (c) 2007 The Electrochemical Society.