The influence of CO2 and steam on the H2 permeation behavior of high-flux Pd/Ag alloy and Pd membranes has been determined. The membranes – fabricated with microsystem technology (MEMS technology) – are 1.0 and 0.5 μm thick; the thin membranes will reveal inhibitive effects more strongly than relatively thick membranes. The H2 flux through the membranes has been studied for periods of more than 80 h during and after addition of CO2 or H2O to the feed. Measurements have been carried out at 623, 673 and 723 K. The largest flux reductions found are 70 and 69% at 623 K for H2O and CO2, respectively. After stopping the steam addition the H2 permeance restores to its original value. After stopping the CO2 addition the H2 permeance increases, but to a value lower than before the CO2 addition. Surprisingly large time constants have been found for the transient permeance behavior. The inhibitive effect caused by the addition of CO2 is almost certainly caused by the formation of CO and H2O by the reverse water gas shift (WGS) reaction. Besides by adsorption of CO2, CO and H2O, a far stronger inhibitive effect is caused by carbon, which is formed by the dissociation of CO and CO2.