C-13 and H-2 NMR relaxation time measurements were performed on four linear styrene-isoprene tetrablock copolymers (SISI) of overall molecular weight about 12 000 g/mol in order to characterize the segmental dynamics of both components. The two isoprene blocks are equal in length as a-re the two styrene blocks. A wide range of temperatures (from 285 to 530 K) and several compositions (volume fraction of styrene: 23%, 42%, 60%, and 80%) were investigated. Previous small-angle neutron scattering (SANS) results by Lodge et al. showed that these materials are homogeneous on length scales much larger than the radius of gyration. The segmental correlation times for both components in SISI were extracted by fitting the experimental data to the modified Kohlrausch-Williams-Watts (mKWW) orientation autocorrelation function and Vogel-Tammann-Fulcher (VTF) temperature dependence. While previous measurements found that the global dynamics in this system are quite homogeneous, these NMR measurements show that the segmental relaxation times of styrene and isoprene segments differ by orders of magnitude. At high temperatures, the dynamics of styrene segments are about 10 times,dower than that of isoprene segments; this can be attributed to the difference in the intrinsic mobility of the two types of segments. The segmental dynamics of 5% polyisoprene (PI) tracers in one tetrablock matrix are very similar to that of the matrix isoprene segments.