The fractional pyrolysis of Bright tobacco was performed in a nitrogen atmosphere over the temperature range of 240-510 degrees C in a specially constructed, high-temperature flow reactor system. Electron paramagnetic resonance (EPR) spectroscopy was used to analyze the free radicals in the initially produced total particular matter (TPM) and in TPM after exposure to atmospheric, dry- or water-saturated air (argon). When the initially produced TPM was allowed to age, the formation of secondary radicals with an EPR g value of 2.0053 +/- 0.0003 was observed. The rate of the formation of these radicals was found to depend dramatically upon the presence of water vapor in the exposure atmosphere. The secondary radicals were nearly undetectable in the water-saturated atmosphere and increased significantly by decreasing the water concentration by partial evacuation or purging with a dry air/argon atmosphere. The EPR spectra and the conditions of the formation of the observed secondary radicals were consistent with many aspects of oxygen-centered, semiquinone radicals formed from hydroxylated aromatic molecules, including hydroquinones and catechols, chemisorbed to metals or other electron-acceptor sites in the TPM. In conjunction with previous studies of primary radicals produced prior to aging of the tobacco, we believe that semiquinone-type radicals are largely produced in aged tobacco smoke and are at much lower concentrations in primary tobacco smoke.