Measurements of the hydrogen permeation rate through a Pd/stainless steel membrane have been carried out in the presence of carbon monoxide and steam and with mixtures of these two additives. Significant reductions in hydrogen permeability were observed, the extent of the reduction being dependent on the amount of CO or steam added. At low concentrations of additive(high feed flux), differences in behavior for CO and steam were noted, addition of CO at this level having little effect on hydrogen permeability, whereas addition of steam had a much more pronounced effect. (C)2000 Elsevier Science B.V. All rights reserved.