A new metformin (Mf) ion selective PVC membrane electrode based on the ion-pair complex of Mf with sodium tetraphenylborate was prepared and its performance characteristics were studied. The electrode exhibited a linear response with a good Nernstian slope over a relatively wide range of concentration. The calibration graph slope was constant for up to 14 days with continuous soaking at 54 mv/ concentration decade at 30°C. Then the slope decreased as the time of soaking increased, reaching 42 mv/decade after 21 days. The changes in pH did not affect the electrode performance within the range 4.0–11.0. The standard electrode potentials were determined at different temperatures and were used to calculate the isothermal coefficient of the electrode (0.0004 V/°C). 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.