Li-rich layered transition metal oxides such as Li[Li0.2Ni0.2Mn0.6]O-2 have in-plane ordering between the excess Li atoms and the transition metal (TM) atoms in the transition metal layer. The root 3a x root 3a superlattice in the TM layer causes superlattice Bragg peaks in their X-ray diffraction patterns. This article describes the relation between the metal composition of the materials, stacking faults and superlattice ordering. The XRD patterns were fitted with a program called FAULTS, which treats the effect of stacking faults on the superlattice peak shapes. The superlattice peak positions of Li[Li1/3-2x/3NixMn2/3-x/3]O-2 materials changed monotonically with Ni content (x), as did the positions of the main diffraction peaks of the base structure. This proves that the superlatices peaks originate from the Li[Li1/3-2x/3NixMn2/3-x/3]O-2 solid solution and are not caused by any domains of second phase such as Li2MnO3. Fitting the XRD patterns with FAULTS revealed that the stacking fault probability increased monotonically with Ni content. (C) The Author(s) 2016. Published by ECS. All rights reserved.