NH3 measurements made using a chemical ionization mass spectrometry (CIMS) method and a molybdenum-oxide, converter-difference (MoOx-CD) approach were compared with those made using the citric acid denuder method. The citric acid denuder, which has fared well in a variety of other evaluations, was used as the benchmark method in the current intercomparison. A very high degree of correlation, r2 > 0.9, between the three ammonia measurement techniques was found. For ammonia mixing ratios greater than approximately 100 pptv, the denuder method and the CIMS technique were found to agree to within about 20%. However, the MoOx-CD measurement results were larger by approximately 70% than the denuder measurements. For our measurements, we conclude that the MoOx-CD method measured the sum of gas-phase NH3 collected by the denuder tube and aerosol ammonium (NH4+) that was simultaneously collected on Teflon filters. The methods and the comparison along with the results of the ancillary measurements are described.