Facilitated co-transport of both silver and copper ions in acidic-thiourea medium has been studied. Thiourea can form successive complexes, which are less mobile than the free metal cation. Tn fact, the mass-transport fluxes of both silver and copper ions decrease when the concentration of thiourea in the feed phase increases. The results obtained have been confirmed by the partition of the different species in the feed aqueous phase Versus the concentration of thiourea by means of a conditional calculation which showed that Ag+ and Cu2+ species predominate at very low thiourea concentration (10(-5)-10(-4) M) and are substituted by complexes M(Tu)(n)(+) (M = Ag, Cu with n = 1-4) with increasing Tu concentration. The fundamental parameters influencing the transport of silver and copper ions have also been determined: Thiourea concentration in the feed phase, pH of the feed-solution, silver and copper concentration in the feed phase, crown-ether concentration in the membrane phase and the membrane support characteristics. It was possible to take advantage of the different chemical characteristics of Ag+ and Cu2+ in presence of thiourea. Working in the right conditions the selectivity of the transport (JAg(+)/JCu(2+)) increased from 1.6 to 5.5 with increasing Tu concentration from zero to 10(-1) M.