The alkaline degradation of cellobiose (4-O-β-D-glucopyranosyl-D-glucopyranose) under oxidative atmosphere at varying pressures (1 (air), 5, and 10 bar) and temperatures (20, 35, and 50 °C) was studied. The degradation was carried out in a microreactor (50 mL) and the samples were taken after a reaction time of 2, 4, 6, 8, and 24 h. Degradation products were determined and identified by gas chromatography-flame ionization detection/mass selective detection (GC-FID/MSD) as their per(trimethylsilyl)ated derivatives. The most prominent oxidized products were glycolic acid (2.3−15.9% of the measured products after 24 h), glyceric acid (0.3−11.4%), 3,4-dihydroxybutanoic acid (2.5−14.6%), and 3-deoxy-pentonic acid (erythro- and threo-forms) (1.6−15.2%) together with minor amounts of tartronic acid, 4,5-dihydroxypentanoic acid, 3,4-dideoxy-hexaric acid (erythro- and threo-forms), arabinonic acid, and several other low-molar-mass aliphatic carboxylic acids. Especially at 20 ^oC an inhibiting character of oxygen upon cellobiose degradation was observed, and more harsher conditions increased the oxidative conversion reactions; at 5 bar and 10 bar after a reaction time of 24 h the content of oxidized products formed at 20, 35, and 50 ^oC were 37.7−43.0, 60.0−62.9, and 27.9−36.8, respectively, of the initial cellobiose. The inhibition and oxidation of cellobiose were kinetically favored, whereas the formation of the main non-oxidized degradation products, lactic acid, α- and β-glucoisosaccharinic acids, and glucose, was formed according to thermodynamical reaction mechanisms. Based on the results a slightly revised ionic reaction mechanism could be suggested.