The phase evolution of Li-rich Li-Mn-Ni-(Al)-O cathode materials upon heat treatments in the air at 900 °C was studied by X-ray and neutron powder diffraction. In addition, the structures of Li_1.26Mn_0.61−xAl_x Ni_0.15O₂, x = 0.0, 0.05, and 0.10, were refined from neutron powder diffraction data. For two-phase mixtures containing a monoclinic Li₂MnO₃ type phase M and a rhombohedral LiMn_0.5Ni_0.5O₂ type phase R, the structures, compositions, and phase fractions change with heat treatment time. This is realized by the substitution mechanism 3Ni²⁺ ↔ 2Li⁺ + 1Mn⁴⁺, which enables cation transport between the phases. A whole-powder pattern fitting analysis of size and strain broadening shows that strain broadening dominates. The X-ray domain size increases with heat treatment time and is larger than the sizes of the domains of M and R observed by electron microscopy. For heat-treated samples, the domain size is smaller for R than for M and decreases with increasing Al doping.