The Thio Schiff-Base (TSB) was used as an excellent ionophore in the construction of a Tm3+ PVC-based membrane sensor. The constructed PVC membrane sensor containing TSB exhibits a Nernstian response of 20.1 ± 0.3 mV per decade for Tm3+ ions over a wide concentration range between 1.0×10-7 and 1×10-2 M, with a limit of detection of 8.6 ×10-8 M. It has a fast response time (<10 s), operates well in the pH range of 3.0–8.7. The best performance was obtained with a membrane composition of 30% poly(vinyl chloride), 55% o-nitrophenyloctyl ether (NPOE), 4% TSB, 10% oleic acid (OA) and 1% sodium tetraphenyl borate (NaTBP).