In this study, a potentiometric titration method for the determination of montelukast sodium in pharmaceutical dosage forms was developed and validated. For this purpose, the potentiometric titration of the standard montelukast sodium was carried out using hydrochloric acid as titrant. The method was found to be highly accurate and precise, having a relative standard deviation of less than 1.0%. From the titration data the stoichiometric protonation constant was calculated in 40% ethanol-60% water and 60% ethanol-40% water (v/v) mixtures at constant temperature of 25.0 +/- 0.1 degrees C and ionic strength of 1.0x10(-1) M NaCl. The protonation constant was found to be 6.25 in 40% ethanol-60% (v/v) water mixture and 5.95 in 60% ethanol-40% (v/v) water mixture. Furthermore, it was shown that the method could be successfully applied to the assay of commercial pharmaceuticals containing 10.0 mg montelukast sodium. The validity of the method was tested by recovery studies of standard additions to a tablet solution and the results were found to be highly satisfactory. This titration method is simple, rapid, accurate, precise and low cost for quality controls of commercial pharmaceutical dosage forms.