Abstract: The use of different ion-pairing reagents in phenformin (I) determination was investigated spectrophotometrically, and WO42- was selected in terms of parameters such as the pH range and reagent concentration. required for best ppt. formation; WO42- formed a white ppt. with I. Flow injection analysis was optimized by using the modified simplex method; the calibration graph was rectilinear over the range 120 to 222 ppm of I (injection volume 275 µL). The method is suggested for use in control determination of I in pharmaceutical formulations.