Oxygen non-stoichiometry in layered rock salt structure cathodes can have a major deleterious effect on their performance in Li batteries. The effect of preparation conditions -temperature, atmosphere and cooling rate - on oxygen non-stoichiometry and consequent interlayer cationmixing in the layered rock salt material LiNi0.8Mn0.1Co0.1O2-delta has been investigated using a combination of X-Ray powder diffraction, including Rietveld refinement and thermogravimetry. Oxygen content varies over the approximate range similar to 0 <=delta <=similar to 0.23; oxygen deficiency is greatest in samples fired at high temperatures in N-2. Interlayer mixing occurs as a direct consequence of oxygen deficiency and leads to reduced cathode capacity and performance; samples with least oxygen deficiency and most cation order were obtained on firing in O-2 at similar to 900-950 degrees C. Attention to the optimisation of oxygen stoichiometry should be an important generic consideration during cathode preparation. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.037204jes] All rights reserved .