Degradation of polyethylene was performed over MCM-41 molecular sieves modified with heteropoly acid H₃PW₁₂O₄₀ (HPW) and its caesium salt CsHPW₁₂O₄₀ (CsHPW). High silica zeolite ZSM-5 has also been used as degradation catalyst. The applied catalysts (HPW/MCM-41, CsHPW/MCM-41 and ZSM-5 zeolite) resulted in lowering of temperature of HDPE degradation when compared to the thermal pyrolysis as well as influenced the distribution of the products formed as a result of the cracking process. HPW/MCM-41 use resulted in low amount of gas products and almost 80 wt.% of liquids while cracking over H-ZSM-5 produced gas hydrocarbons with selectivity about 50 wt.%. Liquid product obtained in the presence of HPW/MCM-41 comprised aliphatic hydrocarbons (olefins and paraffins) while cracking carried out over ZSM-5 zeolite resulted mainly in the formation of aromatic compounds.