CuY and AgY zeolites were studied as selective sorbents for the desulfurization of liquid fuels. Thiophene and benzene were used as the model system, and vapor-phase isotherms were measured. Compared to NaY, CuY and AgY adsorbed significantly larger amounts of both thiophene and benzene at low pressures as a result of pi -complexation with Cu+ and Ag+. On a per-cation basis, more thiophene was adsorbed by Cu+ than by Ag+, e.g., 0.92 molecule/Cu+ vs 0.42 molecule/Ag+ at 2 x 10(-5) atm and 120 degreesC. Molecular orbital calculations confirmed the relative strengths of a-complexation as thiophene > benzene and Cu+ > Ag+. The experimental heats of adsorption for pi -complexation are in qualitative agreement with theoretical predictions.