A series of reactive poly(norbornenylethylstyrene-s-styrene)-poly(w-propyl- p-styrenesul-fonate) (PNS - PSSP) block polymers were prepared by atom transfer radical polymerization. Solutions containing PNS - PSSP, the cyclic olefins dicyclopentadiene and/or cyclooctene, and the second-generation Grubbs metathesis catalyst were prepared, cast as thin films, and allowed to cure at room temperature by a ring-opening metathesis polymerization mechanism. Small-angle X-ray scattering (SAXS) data on cured films were consistent with the formation of nanostructured materials containing PSSP domains confined in a cross-linked matrix of the metathesis-reactive PNS block and the poly(cyclic olefins). The PSSP phase in these films was converted into the sulfonic acid form by hydrolysis of the propyl sulfonate ester. The resulting cross-linked polymer electrolyte membranes (PEMs) were characterized by SAXS and transmission electron microscopy. A bicontinuous morphology with continuous domains of the sulfonic acid phase supported by a continuous and mechanically robust phase was evident in these films. The molecular weight of the PNS - PSSP block polymer controlled the domain sizes, and the mechanical properties of the membranes could be tuned through the choice of cyclic olefins used. The PEMs exhibited pronounced mechanical and thermal robustness. Furthermore, proton conductivities in all the PEMs were similar to those observed in Nafion (the most frequently used PEM in fuel cells) at high humidity. Select PEMs showed significantly lower methanol crossover than Nafion while maintaining high-saturated proton conductivities, which could result in higher direct methanol fuel cell power densities. This reactive block polymer strategy for the preparation of PEMs is attractive due to the ready formation of bicontinuous structures, the facile control of domain size, and the ability to independently control mechanical and swelling properties of the matrix material. 2009 American Chemical Society.