LiNi0.91Co0.09O2 and LiNi0.86Co0.09Mg0.05O2 phases were comparatively cycled in 4/5 ‘A’-size lithium-ion cells (500 cycles, 60°C, C rate, 2h floating at 4.1V at each charge). The MgCo substituted positive electrodes exhibit a very small capacity fading versus the Co substituted one. Comparative Rietveld refinements of the XRD patterns before and after cycling show unambiguously that an irreversible migration of Mg2+ ions from the slabs to the interslab spaces occurs upon cycling. The variation of the cell parameters of the Li x Ni0.86Co0.09Mg0.05O2 deintercalated phases, obtained during the 1st and the 501st charges, were determined. The smaller changes observed after the Mg-migration suggest that the mechanical constraints typically observed during lithium intercalation–deintercalation are reduced by the simultaneous presence of magnesium ions in the lithium sites and of vacancies in the slabs, leading to a smaller capacity fading.