A novel potentiometric method for in-situ testing of terbinafine HCl dissolution behavior using novel liquid inner contact ion-selective electrode membrane was developed. Terbinafine HCl is slightly soluble in water so, the use of ion-selective electrode for its determination is challenge. The sensor was polyvinyl chloride matrix membrane electrode in which sodium tetraphenylborate was used as cation exchanger and bis (2-ethyl hexyl) sebacate as plasticizer. The ion pair was formed by soaking membrane in drug solution. Linear response of terbinafine HCl was found in the dynamic range of 10(-6) to 10(-2) mol L-1. Dissolution test was performed according to FDA regulations using paddle-(USP type 2). So, the proposed ion selective electrode membrane was successfully used as a benchtop in-situ monitor to study the dissolution behavior of terbinafine HCl tablets without need of sample pretreatment or complicated withdrawal steps. Also, the proposed sensor was successfully used for terbinafine HCl determination in dosage forms including spray formulation which contains alcohol percentage as solvent and urine with good recovery. (C) 2018 The Electrochemical Society.