The effect of Co => Ni substitution on the structural and electrochemical properties of LixCo1-yNiyO2 (0 < x < 1, y = 1 or y = 0.8) was studied by a combination of slow scan rate cyclic voltammetry (SSCV) and in-situ XRD techniques. It was shown that LiCo0.2Ni0.8O2 exhibits a single phase region upon oxidation up to 4.08 V (Li/Li+), while LiNiO2 undergoes two phase transitions in the same potential range. At potentials higher than 4.08 V, both compounds exchange Lithium via phase transitions associated with a drastic shrinkage of the interlayer distance in their structures. Long-term cycling between 3.0 and 4.2 V, at full utilization of the active masses, show that the performance of the LiCo0.2Ni0.8O2 electrode is lower than that of LiNiO2, in spite of the homogeneous character of the solid-state reaction in the former compound. As was shown by SSCV and XRD analysis, the decrease of capacity upon cycling in these compounds can be attributed to the loss of the electrode ability of full Li deintercalation. This seems to relate to an appearance of structural defects, such as cation mixing in the Li layers.