High-density polyethylene was functionalized using a thermolysis method in the presence of functionalized peroxides at different temperatures and at various peroxide concentrations. It was found that both percentage cross-linking (% CL) and percentage functionalization (% Fn) increased with an increase in peroxide concentration. The ester and epoxide functionalization was confirmed by FTIR spectroscopy. Ester functionalization was further confirmed by saponification and acidolysis reaction. The functionalization capacities of acrylic ester peroxide and acrylic acid peroxide were determined and compared. A plausible reaction mechanism has been proposed to explain the experimental results obtained. (c) 2005 Wiley Periodicals, Inc.