A routine on-line reverse phase high performance liquid chromatography (HPLC)/HPLC fluorescence method has been developed which greatly simplifies the analysis of selected nitroaromatic compounds in complex matrices. Sensitivity of the method is generally in the low picogram to low nanogram levels. The method is based on on-column catalytic reduction of the non-fluorescent nitroaromatic compounds to the corresponding highly fluorescent amine derivatives. Compound selectivity is achieved by chromatographic separation, use of selective wavelengths for fluorescence detection and column switching techniques. The analytical system conveniently eliminates tedious sample clean-up. In most cases only about 25 micrograms of the crude sample is needed per analysis. For 1-nitropyrene, a precision of ± 3% at the one nanogram level is routinely obtained. Observed 1-nitropyrene levels typically varied from 15 to 120 ppm in the soluble organic fraction of diesel engine particulate emissions and from 4 to 90 ppm in the case of gasoline engine emissions.