New membrane iodide-mercury selective electrode has been developed. The electrode consisted of 20% ion exchanger, tridodesylmethylammoniumiodide (TDMAI), as the active material, 60% PVC as the membrane matrix and dibutylphtalate (DBF) as the plasticiser. An analytically useful potential change occurs from 10(-6) to 10(-1) M iodide ion. and the slope of the linear portion is about 54 mV per decade change in iodide concentration. No interference was observed for SO42-, Cl, Br, F, Cu2+ and Fe-3 ions. The same electrode showed a good response for Hg(II) and a linear relationship was observed in the 10(1) to 10(-5) M concentration range with a slope of 30-32 mV at pH values of 2-4 and 48 mV at pH values of 6-8 per decade, This electrode has been used for the direct determination of iodide in edible salt and for the titrimetric determination of 5 ppm mercury in a coal sample. The lifetime of the electrode was more than 4 months. (C) 2001 Elsevier Science B.V. All rights reserved.