Chemoselective autoxidation of 4-imino-1,5-dimethyl-3-(4-nitrophenyl) imidazolidin-2-one (1b) to its 5-hydroxy derivative 2 occurred in solutions of DMSO-d(6), acetonitrile-d(3) or refluxing ethanol. Also bis(imidazolidin-5-yl) peroxide 5 was isolated as a minor product. It crystallizes as a 1: 1 mixture of R*, R* and R*, S* diastereomers, whereas the NMR spectra of the reaction solution in DMSO-d(6) showed unequal amounts of the two isomers. Molecular mechanics modeling studies with the MM3 force field indicate the R*, S* diastereomer as the more stable one. The 5-unsubstituted and the 5,5-dimethyl substituted imines 1a and 1c, respectively, were found stable against autoxidation; the difference in reactivity of 1b is attributed to the single 5-methyl group enhancing the population of the enamine tautomer. The 5-hydroxy-4-imino-1,5-dimethylimidazolidin-2-one (2) underwent acid hydrolysis to form 5-hydroxyhydantoin 4.