The ESR spectra of radicals produced in Co60γ-irradiated polystyrene have been studied for doses ranging from 1 to 700 megarads. At least three distinct radical species are observed to occur in this dose range. The observed 100 electron volt yield of radicals for 25 °C radiolysis is 0.102 ± 0.005 and is approximately ¼ of the previously reported value. The observed spectrum of the species produced at low doses has been previously reported and interpreted as the disubstituted benzyl radical. The yield for this radical is estimated to be 0.09 per 100 electron volts. The second species, giving a single broad line at g = 2.002, formed with a yield of 0.012, is the most thermally stable species and may involve a crosslink center. This yield is in good agreement with the additional crosslink yield reported to occur when the polymer is heated above the glass transition temperature before exposure to oxygen following vacuum radiolysis. A third radical species is observed at intermediate doses (> 5 Mrad) and is attributed to the chain radical formed by loss of a beta hydrogen. The observed yield of this radical is < 0.002 per 100 electron volts deposited. All three species are observed to decay isothermally at temperatures greater than about 55 °C with the crosslink species having the greatest stability and the beta radical the least. Exposure to oxygen causes the irreversible bleaching of all three species with the same rate. These data are interpreted in terms of the mechanisms of crosslinking, scission and post-irradiative oxidation of polystyrene.