We investigated the segmental and terminal relaxation dynamics of a well-characterized disordered diblock copolymer, poly(isoprene-b-vinyl ethylene) (PI-PVE), and miscible blends of polyisoprene (PI)/poly(vinyl ethylene) (PVE), using dielectric and viscoelastic spectroscopies. Generally, the concentration fluctuation (CF) amplitude of a disordered diblock copolymer is smaller than that of the miscible blend, especially in a length scale longer than the size of the whole block chain. To test whether the difference in the CF amplitudes causes the difference in the segmental relaxation spectra, we compared the shape of the dielectric loss curves between PI-PVE and PI/PVE with the same composition (PI/PVE ratio = 17:83). However, no appreciable difference was observed, indicating that the CF amplitudes in PI-PVE and PI/PVE are not so different in the length scale of the segmental motions. We also examined the effect of distinct friction coefficients of the PI and PVE chains on the terminal relaxation dynamics by comparisons of the viscoelastic and dielectric normal mode relaxations in PI-PVE. The former probes the whole chain motion and the latter probes motions of the PI block. Shift factors (alpha(T)) for the viscoelastic and dielectric relaxations were compared. The dielectric normal mode alpha(T) was found to have weaker temperature dependence than the viscoelastic alpha(T), which indicates that the friction for the PI block chain is lower than the average friction for the PI-PVE chain. (C) 2004 Wiley Periodicals, Inc.