NOVELTY - An ion-conductive membrane (14) comprises a first layer comprising a first ionomer; and a porous polymer substrate, where at least a portion of the first ionomer is interpenetrated within the porous polymer substrate by ionomer-induced phase separation. The porous polymer substrate comprises a polymer having a glass transition temperature of at least about 180degreesC. The ion-conductive membrane has a membrane thickness of less than or equal to 30 (preferably less than or equal to 15 microns).    USE - As electrolyte membranes in a variety of electrochemical applications such as electrochemical devices (e.g. fuel cells), chloroalkali applications, and vapor permeation/separation applications.    ADVANTAGE - The ion-conductive membrane is mechanically reinforced and exhibits good ion conductivity for use in a variety of electrochemical applications. The use of substrate, interpenetrated with ionomers, allows the membrane to have a combination of good mechanical strength and proton conductivity. The ionomer-induced phase separation allows substrate to be filled with ionomer materials in a single-step process, and provides for a more uniform filling of the micropores compared to standard techniques of impregnating ionomers within pre-formed porous polymers. The use of higher equivalent weight ionomers in one of anode layer and cathode layer, and lower equivalent weight ionomers in the opposing layer provides a balance of increased proton conductivity, good mechanical strength, and low swelling with water.    DETAILED DESCRIPTION - An ion-conductive membrane (14) comprises a first layer comprising a first ionomer; and a porous polymer substrate, where at least a portion of the first ionomer is interpenetrated within the porous polymer substrate by ionomer-induced phase separation. The porous polymer substrate comprises a polymer having a glass transition temperature of at least about 180degreesC. The ion-conductive membrane has a membrane thickness of less than or equal to 30 (preferably less than or equal to 15 microns). The ion-conductive membrane further comprises a second layer comprising a second ionomer, where at least a portion of the second ionomer is interpenetrated within the porous polymer substrate by ionomer-induced phase separation. The first ionomer and the second ionomer have different equivalent weights. An INDEPENDENT CLAIM is included for forming an ion-conductive membrane involving coating a first layer adjacent to a second layer, where the first layer comprises a first ionomer dispersed in a first solvent, and the second layer comprises a reinforcement polymer dissolved in a second solvent; and initiating an ionomer-induced phase separation of the reinforcement polymer from the second solvent, so as to form a porous polymer substrate from the reinforcement polymer and interpenetrate a portion of the first ionomer within the porous polymer substrate.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic illustration of a membrane electrode assembly in use with an external electrical circuit.    membrane electrode assembly (10)    electrical circuit (12)    ion-conductive membrane (14)    anode portion (16)    cathode portion (18)    catalyst layers. (20, 22)