Cation mixing during lithium intercalation into and deintercalation from Li1-deltaAlyNi1-yO2 (y = 0; 0.05; 0.25) electrodes was investigated by using galvanostatic intermittent titration technique (GITT) and X-ray diffractometry (XRD). From the XRD patterns obtained from the Li1-deltaNiO2 electrode, the interlayer spacings of the lithium-poorest H3 phase were determined to he smaller than the neighbouring hexagonal H2 phase by 0.03 nm. The cation mixing, i.e., the substitution of nickel ions for lithium vacant sites can account for this shrinkage. The diffusion of lithium ions in the H3 phase was strongly impeded by both the shrinkage of interlayer spacings and the presence of nickel ions in the lithium vacant sites. From the experimental findings that the width of potential plateau at 4.20 VLi/Li+ became narrower with increasing aluminum content, y, it was suggested that the substitutive aluminum ions stabilize the layered structure, thus suppressing the cation mixing.