A PVC membrane electrode selective for trimethoprimium cation based on trimethoprimium-phosphotungstate (TM)3PT ion associate is prepared. The electrode shows a linear response with Nernstian slope over the range of 2.7×10−6-10−2M. Calibration graph slope of the electrode is 59 mV/TM concentration decade when preconditioned by soaking in distilled water for 1 hour to 15 days. The electrode has working pH range of 1.8 to 6.3 and exhibits good selectivity for the trimethoprim with respect to a large number of inorganic cations and organic substances of biological importance. TM is determined successfully in pure solutions and in tablets using the standard additions methods.