An important scientific task is the search for inexpensive and environmentally friendly methods for the production of chitosan oligomers. In the present work, the possibility of chitosan decomposition in the presence of peroxycarbonic acid solutions under high CO 2 pressure was investigated. This medium contains self-neutralizing carbonic acid used here to dissolve chitosan as well as peroxycarbonic acid formed from aqueous hydrogen peroxide, which acts as an oxidizing agent strong enough to break the glycosidic bonds of chitosan. The resulting chitosan decomposition product was analyzed by a number of analytical methods, such as IR spectroscopy, UV-vis spectroscopy, X-ray photoelectron spectroscopy, high resolution mass spectrometry, and X-ray diffraction. It was found that even with relatively low concentrations of hydrogen peroxide and short processing times, it is possible to obtain chitosan oligomers with a molecular weight of about 14 kDa. The obtained polymer was examined by SEM and TG. It showed a good ability to form porous matrices, while its thermal stability somewhat decreased.