Solid-state NMR methods were used to characterize the heterogeneous dynamics, miscibility, and microdomain structure in nanostructured thermoset blends of epoxy resin (ER) and amphiphilic poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymers (PEO-PPO-PEO). NMR experiments show that there is a distinct dynamic difference between the block copolymer (both PEO and PPO) and cured-ER matrix indicating the presence of phase separation, which also confirms the existence of the interphase region including a considerable amount of immobilized PEO and mobilized partially cured ER. An improved method based on spin-diffusion experiments enabled a quantitative determination of the interphase thickness. It is shown that a large percentage of PEO were intimately mixed with ER in the interphase region of the blends, while the rest mobile PEO and all PPO segregated from the ER network. It is observed that the domain size and long period depend strongly on the PEO fraction in the copolymers, whereas the interphase thickness of the blends is not sensitive to the PEO fraction. These NMR results unambiguously demonstrated that PEO blocks were only partially miscible with cured-ER network. Upon curing, the cross-linked rigid ER formed a separated microphase, while some PEO were locally expelled out of the cured-ER network and forms another microphase with PPO. The residual immobilized PEO were intimately mixed with some partially cured-ER matrix and formed the interphase region.