Abstract. We report a new method for the simultaneous in situ detection of nitryl chloride (ClNO₂) and dinitrogen pentoxide (N₂O₅) using chemical ionization mass spectrometry (CIMS). The technique relies on the formation and detection of iodide ion-molecule clusters, I(ClNO₂)⁻ and I(N₂O₅)⁻. The novel N₂O₅ detection scheme is direct. It does not suffer from high and variable chemical interferences, which are associated with the typical method of nitrate anion detection. We address the role of water vapor, CDC electric field strength, and instrument zero determinations, which influence the overall sensitivity and detection limit of this method. For both species, the method demonstrates high sensitivity (>1 Hz/pptv), precision (~10% for 100 pptv in 1 s), and accuracy (~20%), the latter ultimately determined by the nitrogen dioxide (NO₂) cylinder calibration standard and characterization of inlet effects. For the typically low background signals (<10 Hz) and high selectivity, we estimate signal-to-noise (S/N) ratios of 2 for 1 pptv in 60 s averages, but uncertainty associated with the instrumental zero currently leads to an ultimate detection limit of ~5 pptv for both species. We validate our approach for the simultaneous in situ measurement of ClNO₂ and N₂O₅ while on board the R/V Knorr as part of the ICEALOT 2008 Field Campaign.