The (1:1) (phosphomolybdate:metformin) ion pair is used as the ion exchanger in preparing conventional plastic membrane electrodes for metformin (Mf) and its performance characteristics were studied. The electrode exhibited a linear response with a good Nernstian slope over a relatively wide concentration range. The calibration graph slope was constant at 57 mV per concentration decade at 25-degrees-C for up to 5 days with continuous soaking. Then the slope decreased as the time of soaking increased, reaching 42 mV decade-1 after 20 days. The changes in pH did not affect the electrode performance within the range 3.0-10.5. The standard electrode potentials were determined at different temperatures and were used to calculate the isothermal coefficient of the electrode (0.000695 V-degree-C-1). The electrode showed a very good selectivity for Mf with respect to a large number of inorganic and organic cations. The standard addition method and potentiometric titration were used to determine Mf in pure solutions and in pharmaceutical preparations.