H permeabilities (normalized fluxes), have been measured through Pd and some Pd alloy membranes at a series of constant, upstream H(2) pressures with the downstream pressure being similar to 0 in the temperature range from 393 to 573 K. From these data, activation energies for H permeation, E(p), have been determined Conditions of constant upstream p(H2) are of most interest since most determinations of E(p) in the literature have employed this boundary condition. Permeabilities have also been measured at a series of constant upstream H concentrations with the downstream concentration being similar to 0 and, under these conditions, the slopes of the Arrhenius plots give activation energies equivalent to those for H diffusion. It is shown here that under constant upstream p(H2) conditions, nonideality of the H leads to non-linear Arrhenius plots of P for Pd and especially for some Pd alloy membranes where the H(2) solubilities are significant even at moderate pH(2). For example, the permeabilities of a Pd(0.77)Ag(0.23) alloy membrane and a Pd(0.94)Y(0.06) alloy membrane are found to be nearly independent of temperature (423 to 523 K) in the range of constant upstream pressures from 16.1 to 81 kPa.