The relaxation characteristics of a series of random PPS copolymers based on sodium hydrosulfide (NaSH) and 1,4- and 1,3-dichlorobenzene monomers have been investigated via dielectric and dynamic mechanical methods. Comparison of dielectric and dynamic mechanical glass-rubber (α) relaxation temperatures measured for the crystallized copolymers with their amorphous analogues indicated that the presence ofmeta-phenylene defects led to a progressive reduction in the degree of constraint imposed by the crystallites on the relaxing amorphous chains. Estimation of the mobile amorphous phase fraction in the crystallized samples based on dielectric relaxation intensity revealed complete mobilization of the rigid amorphous phase aboveTgfor those copolymers containing 8% and 10%meta-phenylene units. Examination of the subglass (β) relaxation in the amorphous samples showed that the introduction of meta segments in the polymer chain backbone produced a positive offset in relaxation temperatures and a corresponding increase in apparent activation energy. These results indicated that the noncooperative phenylene motions of the β-relaxation occurred more readily in the PPS homopolymer.