We have investigated the gas-phase reaction of the α-aminoacetate (glycyl) radical anion (NH 2 •CHCO 2 -) with O 2 using ion trap mass spectrometry, quantum chemistry, and statistical reaction rate theory. This radical is found to undergo a remarkably rapid reaction with O 2 to form the hydroperoxyl radical (HO 2 •) and an even-electron imine (NHCHCO 2 -), with experiments and master equation simulations revealing that reaction proceeds at the ion-molecule collision rate. This reaction is facilitated by a low-energy concerted HO 2 • elimination mechanism in the NH 2CH(OO •)CO 2 - peroxyl radical. These findings can explain the widely observed free-radical-mediated oxidation of simple amino acids to amides plus α-keto acids (their imine hydrolysis products). This work also suggests that imines will be the main intermediates in the atmospheric oxidation of primary and secondary amines, including amine carbon capture solvents such as 2-aminoethanol (commonly known as monoethanolamine, or MEA), in a process that avoids the ozone-promoting conversion of •NO to •NO 2 commonly encountered in peroxyl radical chemistry. © 2012 American Chemical Society.