The segmental and terminal relaxation processes of polystyrene, styrene-acrylonitrile, and cr-methylstyrene-acrylonitrile copolymers have been investigated by means of both dynamic-mechanical and dielectric spectroscopy in the linear response region. The temperature dependence of the average relaxation time tau of the two processes follows a Vogel-Tamman-Fulcher (VTF) equation: tau proportional to exp[B/(T- T infinity)]. Nevertheless, the segmental and terminal relaxations exhibit appreciably different VTF parameters. This vitiates time-temperature superpositioning in the segmental relaxation temperature region, giving rise to complex thermorheological behavior. As first shown by Plazek et al., this finding further confirms the Donth and Ngai models. Peculiar relationships between the VTF parameters of the segmental and terminal relaxation of the same polymer and of the same relaxation process of different polymers are pointed out. These relationships reveal general features of the VTF equation. A comparison between dynamic-mechanical and dielectric segmental relaxation times (tau (mech) and tau (diel)) highlights a profound difference in the time scales explored by the two techniques. More precisely, segmental motions contributing to the dielectric relaxation are faster than those observed mechanically. The relative magnitude of tau (mech) and tau (diel) was discussed using the DiMarzio-Bishop model. In addition, the ratio tau (mech)/ tau (diel) is found to be temperature-independent. This suggests a sealing law for the decay function phi (t) of the segmental relaxation leading to the same temperature shift factors for different material properties.