Novel tamoxifen-selective potentiometric electrodes based on various ion associations of tamoxifen were developed. The most suitable membrane composition, which has the best potentiometric characteristics, was examined under steady-state conditions. The membrane composed of (w/w) 3% tamoxifen-phosphomolibdate ion pair, 32% poly(vinylchloride), and 65% 2-nitrophenyloctylether was determined as the membrane with the best analytical properties. On the condition of pH = 2-6, the fabricated electrode exhibited linear responses over the concentration range of 9.1 x 10(-6) -1 x 10(-3) M tamoxifen solutions (R-2 = 0.9989) with a sensitivity of 42.9 +/- 0.3 mV/decade, detection limit of 7.3 +/- 0.4 x 10(-6) M and short response time of approximate to 25 s. The influences of some alkaline, alkaline earth metals, biologically significant molecules, and some pharmaceutical species over electrode responses were studied. The proposed electrode was applied for the determination of tamoxifen contents of some pharmaceutical formulations by using standard addition method. The potentiometric analysis results obtained for the pharmaceutical formulations were in good harmony with the results obtained by high performance liquid chromatography at 95% confidence level.