A new liquid membrane based on poly(vinyl chloride) composed of dibutyl(8-hydroxyquinolin-2-yl)methylphosphonate (L) as a neutral ion carrier, sodium tetraphenylborate (NaTPB) as a lipophilic salt dissolved in dioctyl phthalate (DOP) as a plasticizer-cum-membrane solvent was developed for the selective determination of Th4+. The best electrode response for Th4+ is achieved with the membrane composition of PVC:DOP:L:NaTPhB (33%:59%:4%:4%, w/w). The electrode response follows a super-Nernstian response with a slope 31.2 ± 0.4 mV per decade at 25 °C over a wide concentration range of Th4+ from 1 × 10-7 to 1 × 10-1 M. The electrode has a response time of 5 s and detection limit of 1 × 10-8 M, working in the pH range of 4 to 6.5, with a service lifetime of about three months. The selectivity coefficient was determined by the match potential method (MPM) and the results show good selectivity towards Th4+ over a wide variety of selected metal ions (from s, d and f-block metals). The binding mechanism was investigated by FTIR, NMR (1H, 31P) and mass analysis complemented by density functional theoretical results. © 2019 The Royal Society of Chemistry.