The microstructures of styrene-butadiene triblock (SBS) and styrene-butadiene four-arm star block (SB-4A) copolymers and their blends with homopolystyrene (PS) of different molecular weights, MPS, have been investigated by means of small-angle X-ray scattering (SAXS) and C-13 CPMAS NMR. Quantitative analysis with SAXS indicates that the interdomain distance D increases with M(PS) for miscible blends and is independent of M(PS) for partially miscible blends with PS of a much higher M(PS), implying that PS with a much higher M(PS) segregates in between the PS block microdomains as a new homogeneous phase. Meanwhile, the interfacial thickness, t, shows no dependence on the M(PS). The results from proton spin diffusion measurements for blends of SB-4A with PS of a much lower M(PS) indicate that the interdomain size was decreased by the addition of PS, suggesting that there were detectable PS molecules dissolved in the PBD microdomains of SB-4A. Results from both SAXS and NMR are in good agreement for the pure block copolymers but are different for their blends with PS. NMR can give; more reasonable microscopic pictures than SAXS for the studied polymer blends.