The construction and performance characteristics of an ion selective membrane electrode for labetalol drugs in protonated form, based on their ion-pair complexes with sodium tetraphenylborate (TPB-Na+) incorporated in a poly(viny1 chloride) matrix, are described. The Labetalol-H+ selective electrode can be used practically for the determination of the pharmaceutical compound by direct potentiometry. The potentiometric sensor exhibits near-Nernstian response from lo-* M to about M, with a slope of 55 mV/decade of concentration and detection limit of 4.46 X lop6 M. The influence of changing the membrane characteristics, pH, and temperature of the test solution on the electrode potential are studied. The response time varies from a few seconds to several minutes, depending on labetalol concentration. The selectivity of the electrode for Lab-H+ toward primary, secondary, tertiary, and quaternary amines is investigated. Determination of Lab-H+ in pharmaceutical tablets is performed using the proposed membrane electrode. N o interference from any excipients in the tablets was observed.