Recycling is a thermal process in which polymers are melted to produce new products. It is possible that these thermal processes could modify their mechanical and thermal properties. Polymer degradation can be characterized with thermogravimetric analysis and differential scanning calorimetry. Recycled materials tested with these methods have shown variations in some thermal properties, such as the glass-transition temperature and thermal degradation onset, but the sensitivity of these methods is not sufficient to investigate the changes in the characteristics of polymers when materials are exposed to moderate temperature conditions or several thermal cycles. To study these structural changes, a much more sensitive technique, such as pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), is needed. Small variations in the structure can be determined by Py-GC/MS. Each pyrolysis product can be identified by its retention time and mass spectrum with the use of reference literature. In this work, we have studied structural changes in high-impact polystyrene as a function of the injection-molding temperature and pyrolysis cycles. The results do not show significant changes in samples processed at different temperatures with Py-GC/MS, but the values of the pyrolysis products differ as a function of the pyrolysis cycles. (c) 2007 Wiley Periodicals, Inc.