5'-(Tert-butyldimethylsilyl)-2',3'-O-isopropylidene isoguanosine(isoG 1) serves as a selective Cs+ carrier in liquid membrane transport. IsoG~1 is a lipophilic nucleoside that self-assembles via hydrogen bonds and cation-dipole interactions to form a stable decamer sandwich complex with Cs+. Using an acidic receiving phase, Cs+ transport through polymer inclusion membranes (PIMs) was observed at concentrations of isoG 1 below 21 mM. When isoG 1 was precomplexed with Cs+ to give the (isoG 1)10-Cs+decamer, flux was observed above 21 mM carrier. The Cs+ flux increased with increasing carrier concentration of the precomplexed (isoG 1)10-Cs+ decamer. The Cs+ transport selectivity by isoG 1 was investigated in the presence of sodium salt solutions of high concentration. Excellent Cs+ flux and selectivity over the other alkali metal cations was observed in PIMs and bulk liquid membranes (BLMs). In the absence of Cs+, this ionophore exhibitsgood Ba2+ selectivity in BLMs.