The time dependence of the open-circuit voltage (OCV) and the capacity loss in Li/Li1−y CoO2 half-cells during high temperature aging were measured experimentally, and tentatively simulated with simple kinetics equations involving Li-ion diffusion and a phase transition, respectively. The respective activation energies were determined. The capacity loss was found to have a reversible and irreversible component. The latter is discussed in contest of recent observation of an irreversible hexagonal-to-spine phase transition.