In this experimental work the influence of co-existing gases on the hydrogen permeation through a Ti-Ni-Pd membrane was studied. It was found that nitrogen, carbon dioxide and helium do not influence the hydrogen permeation through the dense membrane. However, carbon monoxide influences the hydrogen flux at each temperature investigated (400-500 degrees C). The results show that for low CO concentration (i.e. at H-2 upstream >80%), the hydrogen flux through the membrane decreases faster than linearly, while, at H-2 Upstream <80%, the slope is linear but smaller than the theoretical one. (C) 2007 Elsevier B.V. All rights reserved.