The technique of spin trapping has been applied to the gas phase of cigarette smoke to identify and quantify the radicals present. It was found that radicals could be trapped only if the smoke was filtered. Three spin traps were used: N-tert-butyl-alpha-phenyl nitrone (PBN). 5,5-dimethyl-delta1-pyrroline-1-oxide (DMPO) and alpha-[3,5-di-tert-butyl-4-hydroxyphenyl)-N-tert-butyl nitrone (OHPBN). From the electron spin resonance (ESR) splitting constants of the radicals produced by the reaction of smoke radicals with the spin traps and also from the effec of varying the path length between the cigarette and the spin trap solution, it is concluded that three types of signals are observed. Type I signals indicate the presence of oxygenated radicals which appear to be a mixture of alkoxy radicals (RO) and aroyloxy (ArCO2-) radicals. Our data do not allow conclusions about the nature of the R or Ar groups in these two oxy radicals; however, considerations based on lifetimes suggest that the R group probably is tertiary. Type II and III signals are not typical spectra of spin adducts. Instead, we believe they result from reaction of smoke (and probably radicals in smoke) with the PBN spin trap and indicate that smoke has the ability to effect one-electron oxidations. Only type I signals are observed with DMPO and OHPBN. A quantitative study shows that 4 x 10(14) spins/puff are present in the smoke, in contrast with the result of a recent study which used a very different method for determining the radical content of smoke. A discussion of the nature of the radicals in smoke and some tentative conclusions are presented.