Flowing and static gas-phase samples of HNO3 in 0 2 and N2 were analyzed by long-path ultraviolethisible (TJV/VIS) spectroscopy to reveal the presence of both NO2 and NO3, the concentrations of which were calculated using differential absorption cross sections. NO2 is produced predominantly by the heterogeneous decomposition of HNO3, whereas NO3 is generated in the gas phase by the thermal decomposition of NzO5, a product of the selfdisproportionation of liquid HNO3. 0 1993 John Wiley & Sons, Inc.