Optimizing thin-film Pd-alloy membranes for H(2) purification requires detailed understanding of all possible rate-controlling processes including surface adsorption, solution into the bulk metal, bulk diffusion, dissolution, and desorption. H(2) permeation through Pd(0.77)Ag(0.23) alloy is analyzed by extending pure Pd diffusion models in the literature with a microkinetic model for surface and bulk reactions between H(2) and the alloy. Experimental data and first principles studies from literature are used to estimate and critically modify model parameters. Model results compare extremely well with literature over a broad range of membrane thicknesses (1-20 mu m) and operating conditions. The importance of surface processes increase with decreased film thicknesses (below 10 mu m) characteristic of recent advances in membrane technology. Previously observed enhancement of H(2) flux after heat treatment in air is explained by increased surface area. Sensitivity of H(2) flux to various thermokinetic parameters provides further insight as to when different processes are rate-controlling. (c) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.