A new PVC membrane potentiometric sensor for Ag(I) ion based on a recently synthesized calixarene compound of 5,11,17,23-tetra-tert-butyl-25,27-dihydroxy-calixarene-thiacrown-4 is developed. The electrode exhibits a Nernstian response for Ag(I) ions over a wide concentration range (1.0 X 10(-2) - 1.0 X 10(-6) M) with a slope of 53.8 +/- 1.6 mV per decade. It has a relatively fast response time (5 - 10 s) and can be used for at least 2 months without any considerable divergence in potentials. The proposed electrode shows high selectivity towards Ag+ ions over Pb2+, Cd2+, CO2+, Zn-2(+), Cu-2(+), Ni-2(+), Sr-2(+), Mg-2(+), Ca-2(+), Li+, K+, Na+, NH4+ ions and can be used in a pH range of 2-6. Only interference of Hg2+ is found. It is successfully used as an indicator electrode in potentiometric titration of a mixture of chloride, bromide and iodide ions.