TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Method: At least one of the ionomer layers has a solvent content of 0.5-35% before stage (C). SFP1 and SFP2 may be joined together directly or indirectly via an intermediate membrane; these SFPs show different levels of two-dimensional expansion, so that a projecting edge of an SFP remains after joining the two together. A frame with a projecting edge (see above, 30, 40, 42 or 45) is attached, preferably after combining SFP1 with SFP2 or after applying the first or second ionomer layer and before applying the anode or cathode catalyst layer. The catalyst-coated membrane is attached to a frame comprising two halves of different size, and there may be an intermediate frame between two edges of ionomer layers projecting over the catalyst layers. At least one of the catalyst layers is attached to a gas diffusion layer (GDL), preferably both, so that each combination forms a flush joint or at least one of the GDLs has an edge projecting over a catalyst layer and at least partly overlapping with a frame. The coated membrane is attached to a frame and on both sides to a GDL, with a seal between the membrane and the frame or between the frame and a GDL. Before stage (C), at least one additional layer may be inserted between SFP1 and SFP2, comprising an additive selected from solvents, polyelectrolyte solutions or dispersions, fillers and catalysts. Stage (C) involves combining two SFPs with different degrees of sulfonation in their ionomer layers. At least one of the ionomer layers may contain further component(s) selected from blend components, woven reinforcing fabric, microporous supporting film and fillers. EXAMPLE - Tests were carried out with two membranes of type GK 1065-049d (RTM) (blend membranes made from sPEEK and Ultrason E (RTM); unhydrated) with a residual solvent content (NMP) of more than 22% and a dry film thickness of 22 microns, each on a 100-micron polyester film support. One membrane was sprayed on one side with catalyst ink containing 50% platinum/carbon catalyst and Nafion EW 1100 5% (RTM: ionomer solution) to form an anode-side semi-finished product (SFP1) with a platinum content of 0.15 mg/cm2 and the other was sprayed with similar ink to form a cathode-side product (SFP2) with a Pt content of 0.4 mg/cm2. The film support was removed and the two SFPs were laminated together at 120degreesC to form a membrane with catalyst on both sides. This was treated in 1-N sulphuric acid for 2 hours at 80degreesC, thoroughly washed with water and then pressed together with two gas diffusion layers (SGL Carbon, 21 BC (RTM)) for 10 minutes at 90degreesC under a 20-kN load, to form a membrane-electrode unit (MEA) with an active surface of 32.5 cm2. The MEA was tested in a 25-cm2 cell at 75degreesC, 1bar and 100% RH with hydrogen (lambda = 1.5) and oxygen (lambda = 2). This system (S1) showed a high-frequency resistance of 2.8 m-ohm, compared with 3 m-ohm for a system (S2) in which the MEA was made by coating a single starting membrane (as above but hydrated in sulphuric acid at 80degreesC and containing less than 0.5% solvent) on both sides with catalyst inks as above. The current-voltage curve for S1 was below that for S2 over the range 0-1200 mA/cm2.