The functionalization of polyethylene was realized by the thermal decomposition of peroxyesters in molten polymer in the absence of any other additive. Ester and acid functions were introduced onto the polymer operating with various peroxyesters. This grafting resulted from the combination of a polymer radical, generated in the abstraction of a hydrogen from polyethylene by an alkoxy radical, with an acyloxy or carbon-centered radical arising from the perester. A complete methodology was set up to identify and to titrate the functions present onto the polyethylene and to determine the extent of crosslinked polymer.