A Th 4, ion-selective membrane sensor was fabricated from poly (vinyl chloride).(PVC) matrix membrane containing 2-(diphenylphosphorothioyl)-N',N'-diphenyl acetamide (DPTD) as a neutral carrier, potassium tetrakis (p-chlorophenyl) borate (KTpCIPB) as anionic excluder and o-nitrophenyloctyl ether (NPOE) as a plasticizing solvent mediator. The effects of the membrane composition, pH and additive anionic influence on the response properties were investigated. The sensor, comprising 30% PVC, 63% solvent mediator, 4% ionophore and 3% anionic additive demonstrates the best potentiometric response characteristics. It displays Nernstian behavior (15.2 +/- 0.5 mV per decade) over the concentration range 1.0 x 10(-2)-1.0 x 10(-6) M. The detection limit of the electrode is 6.3 x 10(-7) M (similar to 140 ng/ml). The response time of the electrode is 30 s The sensor can be used in the pH range 3.0-9.0 for about 6 weeks. The membrane sensor was used as an indicator electrode in the potentiometric titration of Th4+ ions with EDTA. It was successfully applied to the determination of thorium ions in binary mixture.