Abstract We have reported that metal chloride-graphite intercalation compounds (GICs) prepared from flexible polyimide film-derived graphite sheets, such as CuCl2-, MoCl5-, and AlCl3-CuCl2-GICs, demonstrate excellent air stability. In this work, the high-temperature thermal stability and gas release properties of those GICs were investigated. MoCl5- and AlCl3-CuCl2-GICs were unstable at high temperatures, exhibiting weight losses at 150 °C and 100 °C, respectively, which accompanied the release of HCl and Cl2 gases. In contrast, the CuCl2-GIC demonstrated relatively high thermal stability in inert atmospheres at temperatures below 400 °C, showing neither weight loss nor gas release. After heat treatment at 600 °C at a heating rate of 5 °C/min, only a slight decrease in the electrical conductivity was observed.