A PVC membrane containing N,N’-bis(pyridine-2-carboxamido)-2-aminobenzylamine (BPCA) as a suitable ionophore, sodium tetraphenyl borate (NaTPB) as an anionic additive, nitrobenzene (NB) as solvent mediator, exhibits a Nernstian response (slope of 20.4 ± 0.3 decade of activity) for Tm(III) ions over a wide concentration range up to 1.0×10–6 and 1.0×10–2 M, with a detection limit of 6.3×10–7 M in the pH range 2.8–8.3. It has a fast response time (~5 s).