A new method for the preparation of asymmetric membranes with an interpenetrating proton-conducting morphology, which consists of cross-linked sulfonic acid functionalized ionomers embedded within a matrix of a thermally-resistant, glassy polymer is presented. This method combines a traditional immersion precipitation process for making membranes with photopolymerization and crosslinking of functional monomers included in the casting solution. The resulting membranes have an integral top skin layer with fine proton-conducting channels on top of a coarser proton-conducting support. In-plane conductivities of some of these membranes measured at ambient temperature were significantly higher than the conductivity of Nafion membranes, while having improved methanol barrier properties. An increase in functionality and molecular weight of crosslinking agents, as well as a selection of materials to promote delayed, rather than instantaneous liquid-liquid demixing were associated with significant improvements in membrane conductivity. Based on the results, a potential use of such membranes in direct methanol fuel cells looks promising. (C) 2012 Elsevier B. V. All rights reserved.