Li1−z Ni1+z F y O2−y (0≤y≤0.2) and Li1−z (Mg x Ni1−x )1+z O2 (0<x≤0.15) were synthesized by partially substituting oxide anions with F− and Ni cations with Mg2+, respectively. The heavily fluoridated samples showed a high degree of cation disorder resulting from the increase in Ni2+ nickel ions at the Li sites. The linear dependence of z on the c h/a h ratio (based on Rougier's work Rougier, Gravereau, Delmas, J. Electrochem. Soc. 143 (1996) 1168) was used to estimate the extent of cation disorder in the layered structure. For lithium magnesium nickel oxides, the substitution has resulted in some oxygen non-stoichiometry. A simple argument based on the z values suggests increasing presence of Mg2+ cations in the Li layer at high degress of Mg substitution. Elemental analysis confirmed the increase in lithium deficiencies with increasing amounts of substituents in the layered structure.