Dilute gaseous standards of CH3ONO in N2 and Ar were used to calculate conformer abundance in the gas phase using FTIR spectroscopy. The standards were composed of varying concentrations of X:CH3ONO, where X = N2 or Ar. Xe was excluded from the gas-phase experiments due to the worldwide shortage in the gas supply. IR transition frequencies corresponding to each isomer were identified based on previous literature, with cis centered at 1623 cm -1 and trans observed at 1680 cm -1 . The abundance of each conformer in the gas phase was quantified by comparing the integrated areas under the peaks corresponding to the fundamental N=O stretch ν3 (see Supplemental Material (SM), Section 2). The average roomtemperature cis and trans conformer relative abundances were determined to be 57 ± 2% to 43 ± 2%, respectively, regardless of bath gas. Previous studies have shown that the ν3 stretch of cis and trans-CH3ONO have approximately the same absorption cross section and we use this assumption here, with further details given in SM, Section S1, to equate absorbance ratios with concentration ratios. Our experimentally determined cis and trans gas-phase abundances are in good agreement with previous observations that used gas-phase IR spectroscopy to determine conformer abundances at room temperature.