The leak (of He gas) in composite Pd membranes prepared by the electroless deposition method was studied. The leak was distributed over the entire surface of membranes, and was due to the formation of pinholes, 0.1-2 mu m in dia., located at the boundaries of Pd clusters and Pd crystallites. The rate at which the leak developed in H-2 atmosphere was determined at 450, 500 and 550 degrees C, and an apparent activation energy for the leak growth was calculated to be equal to 238 kJ/mol, which was close to the activation energy of Pd self-diffusion coefficient of 266 kJ/mol. From experimental data at 450, 500 and 550 degrees C, the leak rate increase at 400 degrees C was extrapolated. In order to corroborate the calculated rate at 400 degrees C, a 4 mu m thick composite Pd membrane, M-53, was studied for over 2,000 h in H-2 atmosphere. M-53 showed a H-2 permeance of 42 m m(3)/m(2) h bar(0.5), and an outstanding selectivity (H-2/He) higher than 22,000 over 2,200 h, which was in agreement with the calculated leak rate increase at 400 degrees C. (c) 2007 American Institute of Chemical Engineers.