The presence of a limited amount of H

2

S in H

2

-rich feed adversely affects the Pd-Cu membrane permeation performance due to the sulphidization of the membrane surface. A theoretical model was proposed to predict the S-tolerant performance of the Pd-Cu membranes in presence of H

2

S under the industrial water-gas-shift (WGS) reaction conditions. The ideas of surface coverage and competitive adsorption thermodynamics of H

2

S and H

2

on Pd-Cu surface were introduced in the model. The surface sulphidization of the Pd-Cu membranes mainly depended on the pressure ratio of H

2

S to H

2

temperature and S-adsorbed surface coverage

i.e.

the occurrence of sulphidization on the surface was not directly related with the bulk compositions and structures [body centered cubic and face centered cubic (bcc or fcc)

]

of Pd-Cu alloy membranes because of the surface segregation phenomena. The resulting equilibrium equations for the H

2

S adsorption/sulphidization reactions were solved to calculate the pressure ratio of H

2

S to H

2

over a wide range of temperatures. A validation of the model was performed through a comparison between lots of literature data and the model calc

ulations over a rather broad range of operating conditions. An extremely good agreement was obtained in the different cases

and thus

the model can serve to guide the development of S-resistant Pd alloy membrane materials for hydrogen separation.