The fractional pyrolysis of Bright tobacco was studied in an inert nitrogen environment at atmospheric pressure over a temperature range of 200-700 degrees C, using a continuous-flow reactor system. The effect of pyrolysis conditions on the generation of free radicals and their distribution among total particulate matter (TPM), char residue, and the gas phase was characterized using electron paramagnetic resonance (EPR) and EPR spin-trapping techniques. It was found that the fractional pyrolysis of tobacco led to the generation of free radicals with g-factors of 2.0035 - 2.0040. They were: formed during pyrolysis at temperatures of > 380 C and were present in both char and TPM. The concentration of these radicals in TPM varied from 0 spins/g at 240 degrees C to 11 x 10(16) spins/g of TPM at 620 degrees C. The concentration of these radicals in the char residue increased as the pyrolysis temperature increased; a maximum concentration of 1.2 x 10(19) spins/g of char at 480 degrees C was obtained, and then the value declined. The g-values of 2.0035 - 2.0040 are consistent with (i) surface-associated, carbon-centered radicals, where the unpaired electron is vicinal to an oxygen-containing functional group, or (ii) partially delocalized, polymeric, phenoxyl-type radicals. The analysis of the gas phase of tobacco smoke revealed the formation of alkoxyl radicals. The N-tent-butyl-alpha-phenylnitrone (PBN) adduct of the alkoxyl radicals is characterized by hyperfine coupling constants of alpha(N) = 13.9 G and alpha(H) = 2.0 G and a g-factor of 2.0064. The concentration of these radicals in the gas phase was insignificant ((5 +/- 1) x 10(14) spins/g of tobacco) and was not dependent on the pyrolysis temperature. After exposure to air, the concentration of TPM radicals increased, because of the production of "new" radicals, in addition to the "old" radicals, with the increasing time of storage, reaching a maximum value and then declining. The g-factor of these new radicals is 2.0053 +/- 0.0003, and their TPM concentration achieved a maximum for a pyrolysis temperature of 280 degrees C. At this temperature, the concentration of these new radicals is 8.1 x 10(15) spins/g of tobacco, which is 25 times greater than the concentration of radicals measured. immediately after collection. The EPR spectra of the new radicals are consistent with oxygen-centered, semiquinone-type radicals. These results suggest that hydroquinone/catechol-type species, which are free or chemically bound to TPM and do not produce EPR signals, are converted to semiquinone-type radicals by atmospheric oxidation.