Methyl β-D-ribopyranoside (1) and methyl β-D-xylopyranoside (2) were degraded by oxygen (0.682 MPa partial pressure) in 1.25Msodium hydroxide at 120°C. The degradations of1and2were similar to the previously reported degradations of 1,5-anhydroribitol (3) and 1,5-anhydroxylitol (4), respectively.4Both hydrogen peroxide and stable organic peroxides were detected in the reactions of1and2. The riboside1degraded faster than the xyloside2. This difference in reactivity is proposed to be a function of the relative acidity of the glycosides. Ionization of hydroxyl groups is postulated to be favored in1, thus facilitating the initiation of the free radical degradation. The degradations of both1and2exhibited complex kinetics indicating autoinhibited reactions. In spite of the differences in reactivity, glycosidic bond cleavage occurred in approximately 60% of the degradations for both1and2. C-1 radicals, resulting from abstraction of the anomeric hydrogen atom, are proposed to cause the observed autoinhibitionviatermination reactions with α-hydroxyhydroperoxyl radicals. However, decomposition of the glycosidesviaC-1 radicals is not believed to constitute a major degradation pathway since the reactivities of1and2were essentially the same as the analogous 1,5-anhydroalditols, i.e.3and4, respectively. The major acidic degradation products of1and2were identical, but were formed in different relative ratios. The major acidic products were methoxyacetic acid, lactic acid, glycolic acid, glyceric acid, a methyl 3-C-carboxyfuranoside, and two isomeric 2-C-carboxyfuranosides.