The thermal autoxidation of cis-1,4-polybutadiene was studied by FTIR at various temperatures. The spectral information obtained gives more details of the autoxidation than was previously available by dispersive infrared spectroscopy. The primary mechanism involves both cyclic alkoxyperoxides that had been previously suggested by Bevilacqua and Bolland, respectively. We have demonstrated that the loss of cis and the formation of trans structures at 25° and 60°C is an oxidative process.