The combination of a tandem column ensemble and an on-line microsorption trap is used for the analysis of organic compounds in human breath samples. The four-bed sorption trap uses a series of discreet sorption beds containing three grades of graphitized carbon and a carbon molecular sieve to quantitatively remove most organic compounds from 0.8-L breath samples. The trap is then heated to 300 degrees C in approximately 1.5 s and maintained at this temperature for 10 s. The resulting vapor plug width is in the range 0.7-1.3 s for the compounds found in the breath samples. The separation is performed with a 15-m-long, 0.25-mm-i.d. capillary using a 0.5-microm-thick film of nonpolar dimethyl polysiloxane coupled in series to a polar column, either trifluoropropylmethyl polysiloxane or poly(ethylene glycol). Both column combinations are successful in separating the early-eluting compounds acetone, isoprene, pentane, methyl alcohol, and ethyl alcohol, which are all common in breath samples. The poly(ethylene glycol) combination gave better separation but showed relatively fast deterioration for repeated analysis of wet samples. Breath samples were obtained under different conditions (smoker, nonsmoker, gum chewer), and 25 compounds were identified in the various samples. Many additional peaks are observed but not identified. Analytical curves (log-log) of peak area versus sample volume for test compounds are linear in the range 80-800 cm3. Detection limits (3sigma) for several volatile compounds in 800-cm3 samples are in the 1-5 ppb range.