Methyl hydroperoxide (MHP) is produced by the CH3O2 + HO2 reaction in the oxidation cascade of volatile organic compounds (VOCs). During KORUS-AQ (May–June 2016), aircraft observations over Seoul using chemical ionization mass spectrometry reported MHP concentrations exceeding 1 ppb in the planetary boundary layer, four times higher than simulated by the GEOS-Chem atmospheric chemistry model. We show that this discrepancy can be explained in part by the instrument's positive interference from methanediol (MD) under high-humidity conditions, where MD is produced in clouds by formaldehyde hydration. Including MD chemistry in GEOS-Chem increases the global formic acid source by 11\% but has otherwise minimal impact on the model chemistry. Observed MHP concentrations in the dry free troposphere are much less sensitive to MD interference and vary with the branching ratio of the CH3O2 reaction with HO2 versus NO, supporting current understanding of low-NO and high-NO chemical regimes for VOC oxidation.