High-density polyethylene (HDPE) was graft functionalized with two different mercaptoesters in an inert atmosphere at 160 degrees C under different experimental conditions by thermolysis method. The order of functionalization, crosslinking, and C=C formation reactions were determined from the relative intensities of carbonyl stretching vibration and C-H bending vibrations. FTIR, DSC, and TGA analytical tools were used to characterize mercaptoester-functionalized HDPE. A plausible reaction mechanism is proposed here to explain the experimental results obtained. POLYM. ENG. SCI., 50:474-483, 2010. (C) 2009 Society of Plastics Engineers