A mercury ion-selective electrode (ISE) was prepared with a polymeric membrane based on a calixarene derivative containing a thiazole azo group as the ionophore. Several ISEs were conditioned and tested for the selection of common metal ions. The ISE gave two kinds of response slope for Hg2+. One, which has twice the Nernstian response for Hg2+, is obtained at pH 6.5 with a slope of 61.1 mV per decade in the concentration range of 7.5 x 10(-6)-5 x 10(-2) Mol l(-1). Another, which has a typical Nernstian response for Hg2+, is obtained at pH 4.0 with a slope of 28.7 mV per decade in the concentration range of 5 x 10(-6)-10(-2) Mol l(-1). The ISEs exhibit stable detection reproducibility and high selectivity towards alkali, alkaline earth and transition metal ions except Ag+. The Hg2+-ISE was successfully applied to determine mercury in a real sample.