The sorption mechanism of 1-butene in dry silver salt-doped perfluorosulfonate membranes and the anion effect on the sigma-pi complexation of Ag+/1-butene were investigated. The interaction energy, Delta E, calculated by the MP2(full)/gen 6d//HF/gen 6d molecular orbital study and the limiting heat of sorption, Delta H-0, obtained from the sorption isotherm had nearly the same tendency; the affinity between the silver salts and the 1-butene molecule follow the order of AgBF4 > AgClO4 > AgNO3 > AgCF3SO3. Both the dependence of the differential heat of sorption, Delta H-s, and the half-standard entropy of sorption, Delta S-s, on the concentration of 1-butene sorbed in the membrane had a minimum, which corresponded to the value of C-m in the BET n-layer adsorption equation. 1-Butene molecules can be sorbed and form multilayers on the Ag+-sites in the membrane.