An amphiphilic poly(ethylene oxide)-block-poly(dimethylsiloxane) (PEO-PDMS) diblock copolymer was used to template a bisphenol A type epoxy resin (ER); nanostructured thermoset blends of ER and PEO-PDMS were prepared with 4,4'-methylenedianiline (MDA) as the curing agent. The phase behavior, crystallization, hydrogen-bonding interactions, and nanoscale structures were investigated with differential scanning calorimetry, Fourier transform infrared spectroscopy, transmission electron microscopy, and small-angle X-ray scattering. The uncured ER was miscible with the poly(ethylene oxide) block of PEO-PDMS, and the uncured blends were not macroscopically phase-separated. Macroscopic phase separation took place in the MDA-cured ER/PEO-PDMS blends containing 60-80 wt % PEO-PDMS diblock copolymer. However, the composition-dependent nanostructures were formed in the cured blends with 10-50 wt % PEO-PDMS, which did not show macroscopic phase separation. The poly(dimethylsiloxane) microdomains with sizes of 10-20 nm were dispersed in a continuous ER-rich phase; the average distance between the neighboring microdomains was in the range of 20-50 nm. The miscibility between the cured ER and the polyethylene oxide) block of PEO-PDMS was ascribed to the favorable hydrogen-bonding interaction. (c) 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3042-3052, 2006.