Nanostructured thermosets were prepared by reaction-induced phase separation (RIPS) using a metathesis-cross-linkable monomer in the presence of a metathesis-reactive block polymer. Mixtures of poly(norbornenylethylstyrene-s- styrene)-b-poly(lactide) (PNS-PLA) and dicyclopentadiene (DCPD) in tetrahydrofuran were cross-linked using the second generation Grubbs catalyst. Small-angle X-ray scattering analysis of a series of cured films containing PNS-PLA with varied PLA block lengths and 0-83 wt % DCPD indicated the presence of nanophase-separated disordered morphologies postcuring in most cases. Scanning electron microscopy of cured films after removal of the PLA phase revealed that a majority of the films exhibited a bicontinuous structure. Mechanistic investigations demonstrated that cross-linking the PNS block into the matrix was vital in preventing macrophase separation, and the morphologies observed were relatively unaffected by variations in the concentration of the catalyst. We propose a general mechanism for the formation of nanostructured DCPD using this protocol and develop critical conditions for the formation of bicontinuous structures based on the extent of overlap of the block polymer.