When the compression set results in Table IV were re-examined in the light of the CLD and ∆torque values in Tables II and III, it was obvious that CLD was the main factor in determining the compression set of the rubber vulcanizate. For example, for Compounds 2–4, the crosslink densities were 297–235 mol/m3, and the compression set of these compounds were 17.51–37.41%, respectively. As the CLD decreased, the compression set went up except for Compounds 12–14 and 17–19. Note however that the silica-filled rubber vulcanizates had much higher crosslink densities (Compounds 31–40; Table III). This was because TESPT is a sulfur-bearing silane and was used as a coupling agent. The silica particles reacted with the rubber chains via the tetrasulfane groups of TESPT and produced additional crosslinks to those monosulfidic ones formed by elemental sulfur