The changes in-the structure and catalytic activity of three iron oxides (alpha -FeOOH, gamma -Fe2O3, and an Fe3O4-C composite) were studied by temperature-programmed desorption/degradation (TPD) and XRD analysis. Thermal desorption analysis was performed for the fresh iron oxide catalysts. Thermal degradation experiments were performed:for 4-phenylbutyronitrile as nitrogen-containing model compound adsorbed over alpha -FeOOH, fresh or prepared by heating; 280, 350, and 400 degreesC. TPD was also performed for acrylonitrile-butadiene-styrene copolymer (ABS) mixed with alpha -FeOOH.XRD analysis was performed after each TPD experiment. Catalytic-degradation of 4-phenylbutyronitrile; over alpha -FeOOH leads to formation of light aliphatic nitriles and styrene derivatives. Increasing the temperature, a-FeOOH catalyst gradually changes its composition and crystalline structure, with decreasing the catalytic activity. The-changes-are accompanied by elimination of water and oxygen and can be schematically represented as follows: a-FeOOH --> alpha -Fe2O3 --> Fe3O4. Iron oxide catalysts act mainly on the-primary products of ABS thermal degradation, but they can also degrade the chain end or branched styrene units in the polymer.