The identification and quantification of hydroxylated nitro aromatic compounds in an ambient air particulate extract are presented here. The isomer substitution observed for these compounds supports the mechanism proposed from smog chamber studies: OH attack on the aromatic compound followed by nitration ortho or para to the OH substitutent. Several hydroxylated nitro polycyclic aromatic compounds have been tentatively identified and quantified in the extract also. Both electron ionization (EI) and negative chemical ionization (NCI) high-resolution gas chromatography/mass spectrometry. (GC/MS) analyses were used to identify these components. Bioassay-directed fractionation through three sequential separations led to detection of the hydroxylated nitro polycyclic compounds in a mutagenic and substantially simplified subfraction of the extract.