The 2,3,4,5,6,7,8,9,10,1 1,12-undecabromocarborane anion, 1-HCB11Br11- (UBC-) has been evaluated for its suitability as an ion exchanger in solvent polymeric membrane electrodes and bulk optodes. Experiments comparing the chemical stability of the perhalogenated carborane anion to that of the best lipophilic tetraphenylborate, 3,5-[bis(trifluoromethyl)phenyl]borate (TFPB-), demonstrated that in the presence of 0.2 M acetic acid TFPB- was completely lost within 6 h, while the concentration of UBC- decreased by less than 10% in the same time period. Thin-film bulk optodes containing BME-44 as potassium-selective ionophore, ETH 5294 as chromo-ionophore, and UBC- as ionic sites exhibited a K+ response similar to analogous optodes containing TFPB-, with comparable selectivities over Na+ and Ca2+. Potentiometric measurements evaluating the selectivity behavior of UBC- in both ionophore-free and ionophore-containing electrodes were performed. Ionophore-free PVC membranes containing UBC- as ion exchanger and either DOS or NPOE as plasticizer also demonstrated selectivity similar to TFPB--containing membranes. Sodium-selective membranes containing the ionophore 4-tert-butylcalix[4]arenetetraacetic acid tetraethyl ester (sodium ionophore X) and UBC- as ionic sites showed a Nernstian response for sodium and selectivity comparable to that found in analogous electrodes containing TFPB-.