All fuel cells utilizing the membrane-electrode assembly have their ion-conductive membrane sandwiched between bipolar plates. Unfortunately, applying conventional techniques to isolated polyelectrolyte membranes is challenging and difficult. A more practical alternative is to use the layer-by-layer assembly technique to fabricate a membrane-electrode assembly that is technologically relatively simple, economic, and robust. The process presented here paves the way to fabricate ion-conductive membranes tailored for optimum performance in terms of controlled thickness, structural morphology and catalyst loading. Composite membranes are constructed through the layered assembly of ionically conductive multilayer thin films atopia porous polycarbonate membrane. Under ambient conditions, a fuel cell using a poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) composite membrane delivers a maximum power density of 16.5 mW cm(-2) at a relative humidity of 55% which is close to that of some commercial fuel cells operating under the same conditions. Further optimization of these systems may lead to new, ultrathin, flexible fuels cells for portable power and micropower applications.