Thermal stability and decomposition mechanism of poly(p-acryloyloxybenzoic) acid (PABA), p-methacryloyloxybenzoic acid (PMBA), and their graft coproducts of PP were studied by differential scanning calorimetry, direct pyrolysis mass spectrometry, and TG/IR system, combined thermogravimetric analyzer, and FTIR spectrometer. The homopolymers and corresponding grafts were found to be stable in nitrogen atmosphere but started to decompose under atmospheric conditions when heated above 230 degrees C. PABA and PAPA-g-PP showed a better thermal stability compared to the other polymer. The degradation proceeded predominantly by decomposition of side groups giving phenol, benzoic acid, hydroxybenzoic acid, carboxylic and carbonyl groups, and by decomposition of phenol into cyclodiene mainly. It was also seen that the degradation path did not greatly changed whether the PABA or PMBA were homopolymers or grafted onto PP but the induction temperature of grafted polymers was seen at some 10-20 degrees C higher. (C) 2007 Wiley Periodicals, Inc.