A detailed structural characterization of deintercalated LixNi1-yMgyO2 (y = 0.05 and 0.10) phases was performed in order to determine the effect of magnesium substitution for nickel on the structural evolution upon cycling. For both systems, a solid solution exists over the entire lithium-composition domain, which shows that substitution of 5% of magnesium for nickel is high enough to suppress the phase transitions observed for LixNiO2 during the cycling process. Rietveld refinement of the XRD patterns of the materials recovered after one cycle with an intermediate floating at high potential (4.35 V) or after 50 cycles (between 2.7 and 4.15 V), gave evidence for the migration of all the Mg2+ ions from the slab to the interslab space during the electrochemical process. This cationic displacement is responsible for the existence of a small irreversible capacity at the end of the first discharge. However, Mg2+ cations do not induce any local collapse of the interslab space as is observed for the LixNi1+zO2 systems when the Ni2+ ions present in the interslab space are oxidized. It is assumed that the presence of Mg2+ ions in the lithium sites reduces the cell parameters changes upon cycling and, therefore, is at the origin of the improvement of the cycling properties observed for the magnesium substituted positive electrode materials.