The mechanism of thermal stabilization of delithiated Li1-xNiO2 by cobalt substitution for nickel was closely studied from the structural point of view by thermogravimetry, X-ray diffraction, and X-ray absorption analysis. Delithiated Li1-xNiO2 with hexagonal (R (3) over barm) or monoclinic (C2/m) structure was decomposed to a spinel phase (cubic, Fd3m) at temperatures around 220 degreesC and then converted to a rock-salt phase (cubic, Fm3m) at higher temperatures. Cobalt substitution of nickel in Li1-xNiO2 stabilized the spinet phase, formed from the thermal decomposition of Li1-xNiO2, and suppressed the decomposition of this spinet phase to a rock-salt phase. While the highly delithiated Li1-xNiO2 was eventually converted to a rock salt phase with NiO structure during heating, Co3O4 spinel structure was locally formed around the cobalt ions in Li1-xNi0.85Co0.15O2. The improvement of the thermal stability of highly delithiated Li1-xNiO2 by cobalt addition could be explained by local formation of Co3O4 spinet structure around the cobalt ions in Li1-xNi0.85Co0.15O2. This spinel structure around the cobalt ions was relatively stable at high temperature and therefore, depressed the decomposition of Li1-xNiO2 to a rock-salt phase.