The accumulation of carboxyl groups in low-density polyethylene (LDPE) and LDPE grafted with itaconic acid on the macromolecules kept in hot water, aqueous electrolytic solutions, ethyl and isobutyl alcohols have been investigated. Simultaneously, a photo-initiated oxidation by UV-radiation has also been studied. It was revealed that functionalization of the polyethylene accelerated its thermal and photooxidation processes in water. The depth which the oxidation front of macromolecules reached in aqueous media was larger than that in air. Alkalisation of the aqueous solutions led to a faster oxidation of functionalised polyethylene, but in acidic media the oxidation behaviour did not in fact change as compared with that in an inert environment. When oxidation was photo-initiated in an aqueous medium the light caused a decisive influence on the acceleration of the process. The results obtained show that such oxidation was accelerated owing to hydroperoxide decomposition initiated by water.