The effect of the hydrogenation of the terminal vinyl groups on the peroxide modification and rheological properties of high-density polyethylene (HDPE) was investigated. The aim of the study was to determine exclusively the effect of the terminal vinyl groups on the peroxide crosslinking and rheological properties of HDPE with one polymer type. This was achieved by hydrogenation of the terminal vinyl groups of a commercial HDPE to obtain an identical material from a structural point of view, which differed only in the nature of the terminal unsaturations, and the comparison of its level of peroxide crosslinking with that of the original polymer. Hydrogenated and unhydrogenated polymer samples were modified at 170 degrees C with different amounts of organic peroxide ranging from 125 to 5000 ppm. Changes in the molecular structure were determined by Fourier transform infrared spectroscopy, size exclusion chromatography, and rheological measurements. Hydrogenation of the terminal groups of the original. polymer significantly reduced the rate of modification or crosslinking. The dynamic viscosity and elasticity increased with the level of peroxide modification. Unhydrogenated samples exhibited rapid increases in viscosity and elastic modulus, whereas their hydrogenated counterparts required about 500% of the amount of peroxide needed for the unhydrogenated sample to attain similar structural changes. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 1942-1951, 2010