The complementary techniques of solid-state NMR and small-angle X-ray scattering (SAXS) have been used to determine the microdomain structures of a series of commercial poly(styrene-b-butadiene-b-styrene) triblock copolymers with different molecular weights. The relative locations of multiple scattering peaks, indirect transform, and Pored analysis have been used to determine the morphology and the sizes of various domains as well as the interfacial thickness between them. Solid-state NMR of H-1 spin diffusion measurements was also used to estimate the microdomain structure parameters by simulating the spin diffusion processes with a model that considers the effects of H-1 spin-lattice relaxation. The interfacial thickness, 2 nm for all five materials, and the interdomain distances, ranging from 30 to 42 nm, from NMR agree well with results from SAXS techniques. There are some discrepancies for some of samples regarding the domain sizes of dispersed polystyrene phases obtained from the above two techniques. These discrepancies may be mainly caused by the limited quality of the fitting procedure using indirect transform methods.