Thermal analysis of polycarbonate and polysulfone has shown each plastic to undergo a two-step decomposition, the first step involving production of high-boiling chain scission and volatile products, the second resulting, in addition, in production of large amounts of carbon dioxide and carbon monoxide. Ten to fifteen volatile products of each plastic have been quantitatively analyzed under several combustion conditions and also in several temperature ranges chosen on the basis of TGA curves. Polycarbonate products include a moderate amount of methane, small amounts of other aliphatic hydrocarbons through C5, methanol, acetaldehyde, and several aromatics. Volatile products account for 40–60% of polycarbonate, with phenol and substituted phenols predominating in the large amount of liquid residue formed during chain scission. Polysulfone produces very similar hydrocarbon products as well as sulfur dioxide, which accounts for 85–96% of the sulfur of the plastic, and carbonyl sulfide, which was only detectable during the temperature range 490°–550°C. Volatile products account for 50–60% of polysulfone.