Abstract: In this paper, PVC membrane electrodes were prepared with thiopental-hexadecyltrioctylammonium iodide (HTOA) and thiopental-hexadecyltriphenylphosphonium iodide (HTPP) ion-pair associates as electroactive materials. The effects of the nature of associate counterion and its concentration in the membrane phase have been studied. The electrode based on thiopental-HTOA membrane shows Nernstian response range from 1 × 10^-1 to 1 × 10^-4M, with an average slope of 56.0 mV/pC. The limit of detection is 1.2 × 10^-5M. The potentiometric selectivity coefficients of various common inorganic and organic ions including those which are similar to thiopental in structure were determined. The 0.01 M NaOH is selected as measuring medium for potentiometric measurements. The electrode was used for determining the thiopental sodium content in some injection preparations. The method is quicker and simpler than the pharmacopeia procedure.