A rapid and sensitive spectrophotometric method has been developed for the determination of Th4+ ions using tetra butyl 2,2′-bipyridine-5,5′-diylbis(methylene) diphosphonate (L) as a chemosensor. The selective complexation of L was examined with various f-metal ions like Th4+, U6+ and Ln's(III) ions and other selected metal ions (from s, p, and d-block elements). The chemosensor L selectively portrayed a naked-eye detectable colour change from colourless to pink with an appearance of a new charge transfer band at 320 nm and 525 nm. No noticeable spectral and colour changes of L were observed with other tested metal ions. This confirms high selectivity of L towards Th4+ ions. The Job's plot delineated the formation of a host–guest complex between L and Th4+ in 1:1 binding stoichiometry. The binding constant for the complexation of L-Th4+ was calculated from Benesi–Hildebrand (BH) plot and was found to be 6 × 103 M−1. The complexing ability L with Th4+ was supported by 31P NMR, FT-IR, UV-DRS, vibrating sample magnetometery and mass spectral data. The limit of detection (LOD) and limit of quantification (LOQ) of L for the analysis of Th4+ was calculated to be 2.02 μM and 6.1 μM, respectively. Finally, the chemosensor L was successfully applied for the quantification of Th4+ contents present in monazite sand, gas mantels and different water samples. © 2017 Elsevier B.V.