We observe differences between the spectra of matrix-isolated CH3ONO and the gas-phase spectra of CH3ONO (See SM, Section 2) that are all typical of matrix-isolation spectroscopy, in addition to the observed differences between the spectra obtained with different matrix hosts; the latter being the focus of this work. First, a single transition of CH3ONO (in this case, the cis-ν3 or the trans-ν3) has its vibrational energy shifted compared to that of the unperturbed gas-phase transition-otherwise known as a "matrix shift." These matrix shifts can depend on: (a) the degree of weak interaction between the matrix material and the guest molecule, and (b) the specific vibrational mode of the trapped molecule. Different vibrational modes can be perturbed to different degrees due to the orientation of the trapped molecule inside the matrix cage, thus introducing a larger (or smaller) matrix shift. However, conformer assignment of the matrix- isolated spectrum of CH3ONO is unambiguous and the matrix shift has no effect on measured abundances. Secondly, the total intensity of a given transition can be split into multiple peaks (as seen most clearly in the top N2 trace of Fig. ) due to the guest molecule being trapped in differing matrix sites, otherwise known as "site splitting." Typically, site splitting is only observed to create secondary peaks within a few cm -1 of the most stable site. We integrate over all peaks within a given transition in order to incorporate all configurations of a given conformer. The final difference between gas-and matrix-isolated spectra, as mentioned previously, is the narrow line width of the matrix-isolated transitions. This is in part due to the majority of the rotational energy being quenched in the matrix, allowing the main spectroscopic feature to be a defined single band and enabling straightforward integration in conformer analysis. We integrated the cis and trans-ν3 transitions in the N2 matrix spectrum in the top trace of Fig. , identifying an increase in the population of the more stable conformer cis-CH3ONO in the matrix relative to that of the gas phase. The cis and trans conformer abundances of the freshly deposited N2 matrix are 65 ± 3% to 35 ± 1%, respectively, when deposited at 20°. This increase in the cis-CH3ONO population by ~15% contradicts previous literature showing that CH3ONO retains its gas-phase conformer abundance; our results indicate that the assumption that conformer abundances in matrix-isolation experiments reflect gas phase abundances may not always hold.