An informal intercomparison of gas-phase nitric acid (HNO3) measuring techniques was carried out. The intercomparison involved two new chemical ionization mass spectrometers (CIMSs) that have been developed for the measurement of HNO3 along with an older, more established filter pack (FP) technique. The filter pack was composed of a teflon prefilter which collected aerosols followed by a nylon filter which collected the gas-phase HNO3. The study was carried out during the late winter and early spring of 1996 at a site located on the western edge of the Denver metropolitan area. Throughout the study the two CIMS techniques were in general agreement. However, under certain conditions the HNO3 levels obtained from the nylon filter of the FP gave values for the gas-phase concentration of HNO3 that were somewhat higher than that recorded by the two CIMS systems. The formation of ammonium nitrate (NH4NO3) containing aerosols is common during the colder months in this area. An analysis of these results suggests that the HNO3 collected by the nylon filter in the FP suffers an interference associated with the disproportionation of NH4NO3 from aerosols containing that compound that were initially collected on the teflon prefilter. This problem with the FP technique has been suggested from results obtained in previous intercomparisons. ¿ 1998 American Geophysical Union