The subject of this work is the degradation of pure polyethylene and modified polyethylene films containing 5% and 8% starch, and polyethylene with pro-degradant additives in the form of şmaster batchş in amounts of 20%. The degradation of polymer samples was studied under marine exposure conditions in the Baltic Sea. The experiment was also performed in laboratory at the ambient temperature, in a liquid medium containing sea water with sodium azide (NaN3) to evaluate the resistance of polyethylene against hydrolysis. The incubation of polyethylene samples took 20 months. The characteristic parameters of environment were measured during the period of degradation and their influence on degradation of polyethylene was discussed. Changes in weight, tensile strength and morphology of polymer samples were tested during the experiment performed.