In the solid solution of PMo12-PVAc，a phenomenon of irreversible thermal sensitivity blac-kening and the formation of deepred component in it which arised from thermal chemical reac-tion at 100℃ or 120℃ was studied by XPS and ESR etc．The process of blackening and the deep red component formation are the result of alcoholysis of PVAc in part tO form VA-VAc copolymer first，due to acid effect of phosphomoIybdic acid． Afterwards，when oxidation-reduction occurred，the phosphomolybdic acid was reduced to form not only a part of blue Mi-xed Valence Complex of molybdenum (Vl，V，IV) by VA-VAc copolymer，but also was rcdu-ced further into Mo (III) in part．The hydroxyl of VA-VAc copolymer was oxidized by phosphom01ybdic acid to produce a carbonyl group．This Mo (III) coordinated with the oxy-gen of the carbonyl group of VA-VAc copolymer to form a deep red component of VA-VAc macromolecular metallic Mo (IlI) complex，which brought about these polymer molecule to bind together．Therefore，the system of PMo12-PVAc is an irreversible thermal blackening ma-terial．