NOVELTY - Producing (P1) an ion exchange precursor resin membrane involves: co-extruding an ion exchange precursor resin with an incompatible polymer to form a multilayer film having a layer of the ion exchange precursor resin supported on a layer of the incompatible polymer; and removing the layer of incompatible polymer from the layer of ion exchange precursor resin to provide the ion exchange precursor resin membrane.    USE - For producing an ion exchange precursor resin membrane used for producing an ion exchange resin membrane useful in an energy conversion electrochemical device such as fuel cell; in an electrolyzer; in a separation process or a sensor; and as an electrolyte in a proton exchange membrane fuel cell (all claimed).    ADVANTAGE - The process produces durable, low-cost proton exchange membranes with reduced thickness that reduces ohmic resistance in a fuel cell and improves the overall performance. The process allows continuous prototyping of a uniform multilayer film, where the layer or layers of the incompatible polymer are removed easily to recover the ion exchange precursor resin membrane. Furthermore, material wasted, due to end effects such as edge trimming to eliminate the edges where the gauge consistency is poor, is reduced as the present process reduces thickness variations that can result from flat die film extrusion. The process results in a very thin uniform membrane of ion exchange precursor resin and thus a very thin uniform membrane of ion exchange resin once the ion exchange precursor resin membrane is converted, in comparison to materials currently commercially available. The ion exchange resin membranes produced have thicknesses of less than 25 microns, preferably 3-25 microns, especially 5-25 microns. The process results in large area, biaxially oriented membranes with good dimensional stability, reduced anisotropy and reduced gas permeability. The membrane produced has a dimensional stability expressed as a wet/dry volume change of less than or equal to 75%, preferably 27-75%, especially 27-35%. The membrane is biaxially oriented, and has an orientation ratio of 1-1.5. Moreover, the membranes are unaffected by removal of the layers of incompatible polymers and the morphology of the membranes reduces fuel cross-over in the membranes when they are used in fuel cells.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) producing an ion exchange resin membrane involving converting the ion exchange precursor resin membrane produced by the process (P1) into the ion exchange resin membrane;    (2) a multilayer film produced in the process (P1); and    (3) an ion exchange resin membrane or an ion exchange precursor resin membrane having a uniform thickness of less than or equal to 25 microns and having an orientation ratio of less than or equal to 1.5.