The 1-methylvinoxy radical (1-MVO) is an important intermediate in the combustion and tropospheric reaction of OH. However, the vibrational structures of this species and its anionic form, 1-methylvinoxide anion (1-MVO-), are not fully known. Thus, in this study, we obtained the infrared (IR) absorption spectra of 1-MVO and 1-MVO- trapped in a solid Ar matrix. 1-MVO- anions were produced by electron bombardment during matrix deposition of Ar containing a small amount of acetone. The anions were destroyed upon irradiation at 675, 365, and 160 nm, although the formation of 1-MVO was only observed upon irradiation at 675 nm. The assignment of the IR bands of 1-MVO- and 1-MVO was based on the expected chemistry upon photoexcitation and comparison of line wavenumbers, relative IR intensities, and D-isotopic shift ratios with those predicted at the B3LYP/aug-cc-pVTZ level of theory.