Barium-18-crown-6 complex was used as an excellent carrier for the uphill transport of mercury as Hg(NO2)(4)(2-) complex ion through a chloroform liquid membrane. In the presence of thiosulfate ion, as a suitable metal ion acceptor in the receiving phase, the carrier can selectively and efficiently transport Hg2+ ion from aqueous solution containing other cations such as Cd2+, Pb2+, Zn2+, Fe2+, Ni2+, Cu2+ and Ag+. The amount of Hg2+ transport across the liquid membrane after 2.5 hr is 94.2 +/- 1.3%, while that of other cations is < 6%.