The microphase structures of two commercial polystyrene-b-poly(ethylene/propylene) diblock copolymers with different compositions and molecular weights have been studied by solid-state NMR and small-angle X-ray scattering (SAXS) techniques. The nearly homogeneous narrow proton line shape of the rubber peaks in the two-dimensional wide-line separation (WISE) spectrum indicates that the interfacial region between the rigid polystyrene and the mobile rubber phases is small compared with the size of the domains. H-1 spin diffusion measurements were used to measure the sizes of various domains and the interface between them using a model that considers the effects of H-1 spin-lattice relaxation. The thickness of the interface determined by NMR is about 2 nm for both copolymers. Interdomain distances measured by NMR agree well with the results of SAXS measurements and increase with the increase of the molecular weight of the copolymers. However, the sizes of dispersed polystyrene domains obtained from NMR are smaller than the values from SAXS. This may be caused by the model differences used in interpreting the data from these two methods. This combined approach greatly increases the reliability of the findings and confirms the validity of the simulation model used in the NMR data analyses.