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Calix[6]arene-hexaacetic acid hexaethyl ester, originally developed as a cesium ionophore, is quite suitable for making a thallium(I) (Tl+)-selective membrane electrode. An electrode constructed using this ionophore, with potassium tetrakis(p-chlorophenyl)borate (20 mol% relative to the ionophore) as an ionic additive and bis(2-ethylhexyl)sebacate as a solvent mediator in a poly(vinyl chloride) (PVC) membrane matrix, showed better selectivity for Tl+ than Cs+ (log kTl+,Cs+Pot=−0.7). Thus, the electrode markedly avoided interference from various inorganic and organic cations, because the ionophore is known to well recognize Cs+. The electrode exhibited a near-Nernstian response to Tl+ in the concentration range of 1×10−6 to 1×10−2 M with a slope of 55 mV per concentration decade in 0.1 M Mg(NO3)2. The limit of detection was 0.3 μM. The electrode could be used over a pH range of 2.5–12. Calix[5]arene-hexaacetic acid hexaethyl ester also exhibited good selectivity for Tl+ and showed much less interference from Cs+ than the corresponding calix[6]arene derivative, though interference from alkali metal ions such as Na+ and K+ was significantly increased. The corresponding calix[6]arene and calix[5]arene derivatives with tert-butyl groups were less effective for recognition of Tl+. [Copyright &y& Elsevier]