Facilitated counter-tran sport of silver and copper ions, both in acidic thiourea medium, across a supported liquid membrane (SLM) by using di(2-ethylhexyl)phosphoric acid (D2EHPA) as carrier, dissolved in chloroform, has been investigated. The fundamental parameters influencing the transport of silver and copper ions have been determined. They are: thiourea concentration in the feed phase, pH of the strip solution, D2EHPA concentration in the membrane phase, silver and copper concentrations in the feed phase, pH of feed solution, temperature of the system, membrane support characteristics and the type of acid. Thiourea can form, with both ions, successive complexes, which are less mobile than the free metal cation. In fact, the mass transport flux of silver and copper ions decreases when the concentration of thiourea in the feed phase increases. The obtained results show that Ag(I) and Cu(II) species predominate at very low thiourea concentration (10(-5)-10(-4) M) and they are substituted by complexes M(Tu)(n)(+) (M = Ag, Cu with n = 1-4 and Tu refers to thiourea) with increasing thiourea concentration. Transport fluxes of silver and copper ions from feed solutions of different anionic composition, in absence and presence of thiourea, followed the order: NO3- > Cl- > SO42- > PO43-. This means that preferentially nitric acid should be used in preparing leaching solutions of polymetallic ores that are then used as feed in the SLM system. It was possible to take advantage of the different fundamental parameters for the separation of Ag(I) and Cu(II) in presence of thiourea, indeed working in the optimal conditions, the selectivity of the transport (JAg(I)/JCu(II)) increased from 1.29 to 1.96 with increasing thiourea concentration from 0 to 0.1 M. (C) 2002 Elsevier Science B.V. All rights reserved.