This paper presents a methodology for the preparation of a new active component for ion-selective membranes, based on a di-substituted sulfonium derivative of the closo-decaborate anion at the apical vertices with octadecylalkyl substituents 1,10-B10H8(S(C18H37)2)2. This approach is characterised by physicochemical methods of analysis (11B, 1H, 13C NMR spectroscopy, IR spectroscopy and elemental analysis). The compound obtained was used as an active component of a PVC-membrane selective to terbinafine hydrochloride. The sensor developed is highly selective to the drug to be detected, and has a linearity range of 4.0 × 10-8 – 1.0 × 10-2 and a detection limit of 1.0 × 10-8, and can detect terbinafine hydrochloride in the pH range of 3 to 6.