A tunable diode laser absorption spectrometer (TDLAS) has been used to make real-time measurements of NO/sub 2/ and HNO/sub 3/ in diesel engine exhaust. Teflon filters in the sample line protected the optics from exhaust particulates. Nitric acid and NO/sub 2/ were measured in the range of 40-800 ppbv and 1-30 ppmv, respectively. The mixing ratio of HNO/sub 3/ was found to decrease as engine load increased and to increase with engine speed. The HNO/sub 3/ levels exhibited slow response (10-30 min) both for changes in engine operating condition and when controlled spikes of HNO/sub 3/ were added to the diluted exhaust gas flow. NO/sub 2/ concentrations reached steady values within 5 min of changes in operating conditions. The data indicate that the observed HNO/sub 3/ is present in the exhaust stream immediately after dilution and is not produced by subsequent reactions of NO/sub x/. There was no correlation between the variations in particulate nitrate and the gas-phase HNO/sub 3/ with load and engine speed. 8 references, 5 figures, 2 tables.