H diffusion constants have been determined from steady-state fluxes through Pd-Ag alloy membranes. The upstream side is maintained at a nearly constant pup (and consequently at a nearly constant r(up) = H/(Pd(1-x)-Ag-x) atom ratio, whereas the downstream side is at P-H2 approximate to 0 (r(down) = 0) (423 -523 K). It is shown that the permeability is a maximum for atom fraction Ag, X-Ag = 0.23 (423 -523 K) at both p(up) = 20.3 and 50.6 kPa. D-H has been determined for some Pd-Ag alloys as a function of r in the dilute region, and it decreases with r even at small H contents for alloys with XAg < 0.35. The concentration dependence of DH(CH) has been determined for the Pd0.77Ag0.23 alloy over a large concentration range. The effect of nonideality on DH(r) and E-D(r) has been systematically determined as a function of XAg, where XAg is the atom fraction of Ag in the H-free alloy. (dD(H)/dr) increases with XAg up to XAg = 0.35 and then changes from negative to positive at approximate to 0.35. The activation energies for diffusion, ED(r), have been determined as a function of H content in the dilute range for several Pd-Ag alloy membranes, and the conversion to concentration-independent E-D* values has been carried out in several different ways.