High density polyethylene was hydrothermally cracked in supercritical water using (H2O)-H-2-O-18 and (H2O)-H-1-O-18 as tracers. The objective was to reveal the participation of molecules of supercritical water in the decomposition of polyethylene. It is found that hydrogen atoms from the supercritical water are donated to produce cracked oils, and the oxygen atoms are donated to the gases and aqueous phase products. With an increase in the water fill rate, more hydrogen atom from the supercritical water are transferred to the oils. A mechanism of hydrogen participation in cracking polyethylene is proposed, namely that lower 1 -alkenes (propylene, etc.) decomposed from polyethylene are hydrated to produce secondary alcohols (2-propanol, etc.), which are then oxidized into ketones (2-propanone, etc.) and active hydrogen atoms are liberated. These hydrogen atoms are further donated to produce decomposition products.