Liy(Ni0.425Mn0.425Co0.15)0.88O2 materials were synthesized by a slow rate electrochemical deintercalation from Li1.12(Ni0.425Mn0.425Co0.15)0.88O2 during the first charge and the first discharge in order to study the structural modifications occurring during the first cycle and especially during the irreversible “plateau” observed in charge at 4.5 V vs Li+/Li. Chemical Li titrations showed that the lithium ions are actually deintercalated from the material during the entire first charge process, excluding the possibility that electrolyte decomposition causes the “plateau”. Redox titrations revealed that the average transition metal oxidation state is almost constant during the “plateau”, despite further lithium ion deintercalation. 1H MAS NMR data showed that no Li+/H+ exchange was associated to the “plateau” itself. Rietveld refinement of the XRD pattern for a material reintercalated after being deintercalated at the end of the “plateau”, as well as redox titrations, revealed an M/O ratio larger than that of the pristine material, which is consistent with the oxygen loss proposed by Dahn and coauthors for the LiNixLi(1/3−2x/3)Mn(2/3−x/3)O2 materials to explain the irreversible overcapacity phenomenon observed upon overcharge. X-ray and electron diffraction showed that the transition metal ordering initially present within the slabs is lost during the “plateau” due to a cation redistribution. To explain this behavior a cation migration to the vacancies formed by the lithium deintercalation from the transition metal sites (3a) is assumed, leading to a material densification.