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 10⁻¹M iodide ion, and the slope of the linear portion is about 54mV 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 10⁻⁵M concentration range with a slope of 30–32mV at pH values of 2–4 and 48mV 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 5ppm mercury in a coal sample. The lifetime of the electrode was more than 4 months.