The hydrogen flux in palladium membranes acquired from different vendors has been measured as a function of membrane thickness, temperature and feed (pure H-2) pressure p(1), with permeate (pure H-2) pressure p(2) fixed at 1 atm. Palladium foils 12.5,25 and 47 mum thick were studied at 75-500degreesC and p(1) values of 3.4-6.8 atm. Arrhenius plots of flux, measured with p(1) fixed, include a peak within an intermediate temperature range, and different activation energies in the low and high temperature ranges. The peak may be explained by palladium hydride's alpha-beta phase transition, which, at intermediate temperatures, takes place at hydrogen activities between those at the respective membrane surfaces. The two activation energies apparently reflect different rate-controlling processes: bulk diffusion at high temperatures and surface phenomena at low temperatures.