The objective of this study is to understand and quantify the effects of the reactant and product species of the methanol steam reforming reaction (CH3OH, H2O, CO2, CO) on the H-2 flux through a Pd77Ag23 membrane. Various concentrations of said gases along with H-2 were fed to a membrane separator apparatus containing a 3.9 mu m thick Pd-Ag (23 wt % Ag) "nanopore" membrane. The decrease in H-2 flux through the membrane due to the presence of these gases was quantified at different temperatures (225-300 degrees C) and pressures (3-5 bar). The data show that CO causes the largest drop in H-2 flux while H2O has the least effect. A mechanistically based adsorption and reaction model was developed to quantify the fractional surface coverages of the non-H-2 species. Estimates of surface parameters such as adsorption equilibrium constants and binding energies are consistent with literature values. The adsorption model was incorporated into a two-dimensional separator model that accounted for concentration polarization (radial transport) effects. The model simulations successfully captured most of the trends in the flux data. The developed flux model is suitable for incorporation into a Pd-Ag membrane reactor model in order to evaluate the potential of a methanol membrane reformer for coupled hydrogen generation and purification.