Abstract. A chemical ionization mass spectrometry (CIMS) method utilizing a reagent gas mixture of O₂, CO₂, and CH₃I in N₂ is described and optimized for quantitative gas-phase measurements of hydrogen peroxide (H₂O₂), methyl peroxide (CH₃OOH), formic acid (HCOOH), and the sum of acetic acid (CH₃COOH) and hydroxyacetaldehyde (HOCH₂CHO; also known as glycolaldehyde). The instrumentation and methodology were designed for airborne in situ field measurements. The CIMS quantification of formic acid, acetic acid, and hydroxyacetaldehyde used I⁻ cluster formation to produce and detect the ion clusters I⁻(HCOOH), I⁻(CH₃COOH), and I⁻(HOCH₂CHO), respectively. The CIMS also produced and detected I⁻ clusters with hydrogen peroxide and methyl peroxide, I⁻(H₂O₂) and I⁻(CH₃OOH), though the sensitivity was lower than with the O₂⁻ (CO₂) and O₂⁻ ion clusters, respectively. For that reason, while the I⁻ peroxide clusters are presented, the focus is on the organic acids. Acetic acid and hydroxyacetaldehyde were found to yield equivalent CIMS responses. They are exact isobaric compounds and indistinguishable in the CIMS used. Consequently, their combined signal is referred to as the acetic acid equivalent sum. Within the resolution of the quadrupole used in the CIMS (1 m∕z), ethanol and 1- and 2-propanol were potential isobaric interferences to the measurement of formic acid and the acetic acid equivalent sum, respectively. The CIMS response to ethanol was 3.3 % that of formic acid and the response to either 1- or 2-propanol was 1 % of the acetic acid response; therefore, the alcohols were not considered to be significant interferences to formic acid or the acetic acid equivalent sum. The multi-reagent ion system was successfully deployed during the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) in 2014. The combination of FRAPPÉ and laboratory calibrations allowed for the post-mission quantification of formic acid and the acetic acid equivalent sum observed during the Deep Convective Clouds and Chemistry Experiment in 2012.