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The thermal degradation of ammonium polyphosphate (APP), a commercial fire retardant, and its blends with poly(methyl methacrylate) (PMMA) have been studied by thermal volatilization analysis (TVA) and the degradation products identified. APP degrades under vacuum in three stages. Initially it condenses to an ultraphosphate (<260°C) with release of ammonia and water. Fragmentation follows (260–370°C), giving high‐boiling ammonium salts of phosphate fragments and further ammonia and water. The polyphosphoric acid (PPA) which remains then undergoes extensive Fragmentation (>370°C). In the presence of APP, the normal depolymerization of PMMA to monomer competes with degradation reactions which form high‐boiling chain fragments, methanol, carbon monoxide, dimethyl‐ether, carbon dioxide, hydrocarbons, and char. These additional reactions are initiated principally by the PPA. Intramolecular cyclization occurs, resulting in the formation of anhydride, and ester groups are eliminated, methanol and carbon monoxide being evolved. Further degradation of the modified polymer leads to the other volatile products and the char.