The microphase structure of a series of poly(styrene-ethylene oxide) (SEO) block copolymers has been studied by solid-state NMR and small-angle X-ray scattering (SAXS) techniques. The differential scanning calorimetry studies indicate that the majority of SEO samples are semicrystalline. For the completely amorphous SEO, solid-state NMR TK spin diffusion was used to determine the sizes of various domains, interfacial thickness, and interdomain distances by the initial-rate approximation and by numerical simulation with a pure spin diffusion model. The results obtained from the above two methods, the initial-rate approximation and the numerical simulation, are generally in good agreement, but the interdomain distance estimated from NMR is relatively lower than that obtained From SAXS. The discrepancy is probably caused by the simulation model used in the NMR data interpretation. For the semicrystalline SEO samples, the relative locations of multiple Bragg peaks and the location of the first-order maximum in the SA;YS data were used to determine the morphology and the size of the interdomain distance of the material. This is a typical example where either NMR or SAXS alone cannot yield complete phase structure information for both amorphous and semicrystalline SEO materials. A combination of the above two techniques is necessary and allows us to explore the phase structure of both the amorphous and the semicrystalline SEO samples and also examine the validity of the pure spin diffusion model used for the NMR analysis.