Oxidative RNA damage is linked to cell dysfunction and diseases. The present work focuses on the in vitro oxidation of 5-methylaminomethyl uridine (mnm5U), which belongs to the numerous post-transcriptional modifications that are found in tRNA. The reaction of oxone with mnm5U in water at pH 7.5 leads to two aldonitrone derivatives. They form by two oxidation steps and one dehydration step. Therefore, the potential oxidation products of mnm5U in vivo may not be only aldonitrones, but also hydroxylamine and imine derivatives (which may be chemically more reactive). Irradiation of aldonitrone leads to unstable oxaziridine derivatives that are susceptible to isomerization to amide or to hydrolysis to aldehyde derivative.