The oxygen content in a selection of layered rock salt phases in the system Li–Mn–Ni–O has been investigated by thermogravimetry. Phases that are oxygen-stoichiometric, such as Li2MnO3 and LiMn0.5Ni0.5O2, may lose up to 1% of their oxygen reversibly, on heating to ∼1000°C. Cation-deficient phases, such as Li2/3(Mn2/3Ni1/3)O2, however, are capable of much greater reversible oxygen loss, leading eventually to an oxygen-stoichiometric cubic rock salt phase with a cation-disordered structure. The oxygen contents of such layered rock salt phases are determined directly by their synthesis conditions and have a direct effect on the charge/discharge capacity during lithium deintercalation.