Permeation dynamic of hydrogen through a palladium (Pd) membrane in an environment of varying pressure is investigated and analyzed experimentally. By monitoring the instantaneous pressure and mass transfer rate of hydrogen in the conducted system, the present study provides a comprehensive and precise measurement on the permeance of the membrane. It is found that a threshold of pressure difference between the both sides of the membrane for hydrogen permeation is exhibited. That is, when the driving force of the mass transfer is below the minimum pressure difference, hydrogen permeation will not occur. Accordingly, a modified equation accounting for the hydrogen permeation flux through the membrane is suggested. As a whole, the hydrogen permeation flux versus the pressure difference is characterized by a linear relationship, regardless of what the pressure exponent is. Nevertheless, the optimal pressure exponent is located between 0.5 and 0.7. A dimensionless time, the permeation number, is derived to describe the permeation process. The characteristic time of hydrogen permeation depends on the pressure exponent. The experiments reveal that the permeation number is around 7-13 for the hydrogen permeation flux in the system reaching the quasi-steady state. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.