The influence of typical syngas CO concentrations (20–33%) on the stability of 3 μm thick Pd membranes was investigated between 593 and 773 K in the limit of maximum H 2 extraction. Two kinds of CO inhibition effects were observed: CO adsorption and carbon deposition resulting from catalytic decomposition of CO on the Pd surface. CO adsorption reduced the H 2 permeation rate in particular below 623 K due to reversible blocking of Pd surface sites. Carbon deposition became more severe at higher temperatures and feed gas velocities and accumulated with time. In particular, the formation of a Pd 1− x C x phase was observed above 723 K with x = 0.1. The propensity to PdC formation increased with temperature and the CO concentration in the retentate. Although the carbon incorporated into the Pd membrane could be effectively removed by H 2 and O 2 at 723 K, an irreversible increase of the N 2 flux after the removal of the carbon was observed in particular after treatment with O 2.