LiNiO2 is a promising cathode material for secondary lithium batteries with a reversible capacity of over 200 mAh g(-1). However, a low cycle efficiency of about 80% is observed in the first charge-discharge cycle. To explain this irreversibility, we have assumed a model in which part of the cathode domain becomes electrochemically inactive before the first discharging starts, while the rest remains electrochemically active throughout the cycle. By this model, the active domain is shown to have excellent reversibility. The reversibility in the first cycle can be improved by limiting the charge capacity. An application of this model to a LiCoO2 cathode is also described.