This work reports the preparation and characterization of ultra-thin (similar to 1 mu m thick) Pd-Ag supported membranes for hydrogen purification. Ultra-thin membranes with different thicknesses (ranging from 0.46 to 1.29 mu m) have been prepared by electroless plating (ELP) technique onto asymmetric tubular porous alumina supports. The membranes have been characterized by single gas and mixed gas permeation experiments at temperatures between 300 and 500 degrees C obtaining a correlation for the membrane permeation as a function of the activation energy and the membrane thickness. Hydrogen permeation results of the ultra-thin Pd-Ag membranes have been compared with other highly permeable membranes reported in the literature and they show some of the highest H-2 permeance values. A 1.29 mu m thick membrane has been tested for 1000 h at 400 degrees C and has shown a stable H-2 permeance of 9.0-9.4x10(-6) mol m(-2) s(-1) Pa-1 with a H-2/N-2 perm-selectivity between 3300 and 2000 at 100 kPa transmembrane pressure difference. The same membrane has been tested with a feed gas mixture of H-2/N-2/CO with a 15% CO content and H-2 binary mixtures containing N-2, CH4 and CO2. When tested in a catalyst fluidization environment during 100 h, the 1.29 mu m thick membrane showed stable H-2 permeance and H-2/N-2 perm-selectivity.