In this paper, a poly(vinyl chloride) (PVC) membrane electrode is prepared for gemfibrozil, 2, 2-dimethyl-5-(2,5-xylyloxy) valeric acid, based on its ion pair complexes with hexadecyltrioctyl ammonium iodide (HTOA). The membrane composition of the electrode was optimized by using the sequential level elimination method for orthogonal experimental design. The electrode has a Nernstian response range from 2.5 x 10(-5) to 0.1 mol/l with an average slope of 55.3 mV/decade. The limit of detection is 7.1 x 10(-6) mol/l. The electrode responses were not affected by pH in the range 10.0-12.3. A Na2B4O7-Na2CO3 buffer of pH = 11.0 was selected as the background electrolyte solution for potentiometric measurements. The electrode was used for determining gemfibrozil in pharmaceutical preparations with satisfactory results.