The thermal instability of the cathode materials in lithium-ion batteries is an important safety issue, requiring the incorporation of several approaches to prevent thermal runaway and combustion. Systematic studies, using combined well-defined in situ techniques, are crucial to obtaining in-depth understanding of the structural origin of this thermal instability in overcharged cathode materials. Here time-resolved X-ray diffraction, X-ray absorption, mass spectroscopy, and high-resolution transmission electron microscopy during heating are combined to detail the structural changes in overcharged LixNi0.8Co0.15Al0.05O2 and LixNi1/3Co1/3Mn1/3O2 cathode materials. By employing these several techniques in concert, various aspects of the structural changes are investigated in these two materials at an overcharged state; these include differences in phase-distribution after overcharge, phase nucleation and propagation during heating, the preferred atomic sites and migration paths of Ni, Co, and Mn, and their individual contributions to thermal stability, together with measuring the oxygen release that accompanies these structural changes. These results provide valuable guidance for developing new cathode materials with improved safety characteristics.