Thermoset materials containing ordered structures with similar to 10-nm dimensions were prepared from a mixture of a low-molecular-weight poly(ethylene oxide)-poly(ethylene-alt-propylene) (PEO-PEP) diblock copolymer and a poly(Bisphenol-A-co-epichlorohydrin) epoxy resin that selectively mixes with the PEO block. The phase behavior of PEO-PEP/epoxy blends, with compositions spanning 10-93 wt % block copolymer, was investigated in the uncured state (without hardener) using small-angle X-ray scattering (SAXS) and dynamic mechanical spectroscopy. Without hardener, the phase behavior of block copolymer/epoxy blends was similar to model block copolymer/homopolymer blends and varied with changes in the blend composition and temperature: The following morphologies were observed with increasing epoxy concentration: lamellar, cubic bicontinuous, hexagonally packed cylinders, body-centered cubic packed spheres, and disordered micelles. Methylene dianiline, an aromatic amine hardener, was added to the blends, and the real-time evolution of the phase behavior with cure was followed using SAXS measurements. As the epoxy molecular weight increased, the PEO block segregated from the epoxy matrix, as indicated by an increase in the principal spacing of the ordered structures and the occurrence of order-order phase transitions at certain compositions. However macrophase separation between the epoxy and block copolymer did not occur. These results are interpreted as a transition from an equilibrium morphology to a chemically pinned metastable state as the cross-linking reaction progresses through the gel point.