5,11,17,26,27,arene (1) has been used for the preparation of a graphite coated thorium ion-selective electrode (Th 4+ -ISE). The plasticized PVC membrane containing 30% PVC, 58% ortho-nitrophenyloctylether (NPOE), 4% sodium tetraphenylborate (NaTPB) and 8% ionophore was directly coated on a graphite rod. This sensor gave good Nernstian responses with a slope of 15.5 ± 0.1 mV/decade over a concentration range of 1´10 -5 -11 0 -3 M of thorium ions with a limit of detection of 7.9´10 -6 M. The dynamic response time of the electrode to achieve a steady potential was found to be about 15 seconds. The potential of the prepared sensor was independent of the pH variation in the range 2.3-4.0. The selectivity relative to several mono-, di-and tri-valent metal ions, i.e. Li + , Na + , K + , Ag + , NH 4 + , Sr 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ , La 3+ , Sm 3+ , Dy 3+ , Er 3+ and Y 3+ was examined. This electrode can be used for 6 months without any considerable divergences in the potential response. The sensor was successfully used as an indicator electrode for the potentiometric titration of a thorium solution using a standard solution of EDTA.