Fick's diffusion constants, D-H, for H in Pd-Y alloys decrease with the increase of atom fraction Y, X-Y, for the alloys investigated here (X-Y <= 0.095), however, the H permeabilities increase because the H-2 solubility increase is greater than the decrease of the diffusion constant. DH values have been measured for some of the Pd-Y alloys over a large range of H contents and, for several alloys, minima appear in plots of RT ln DH against r(up) from non-ideality, i.e., thermodynamic factor not equal 1, where r(up) = H-to-metal mol ratio at the upstream surface of the membrane and r(down) = 0. Concentration independent diffusion constants have been derived from the concentration-dependent ones and the thermodynamic factor. The effect of CO on the permeation of H through Pd-Y membranes has been determined and compared to the effect of CO on the permeability of a Pd0.77Ag0.23 alloy. The poisoning effect of CO is either greater or similar for the un-oxidized Pd0.77Ag0.23 alloy membranes and the internally oxidized Pd-Y alloy membranes, although any differences are small. (C) 2015 Elsevier B.V. All rights reserved.