Polymer-modified electrodes are one of the most rapidly growing areas of modern electrochemistry. These electrodes have aroused intense interest due to their inherent advantages such as simple and economic fabrication, fast and sensitive response, low resistance and high antifouling performance. A number of sensors have been developed for the direct potentiometric determination of ions in aqueous solution based on a correlation between a selective response and doping of the anion in the polymers. We report herein a new sensor modified with poly(methylthiophene-methylpyrrole) copolymer. This modified electrode shows near Nernstian response for Br−over a concentration range of 10−4to 10−1M. The electrode response also exhibits a sharp endpoint response for the potentiometric titration of Br−.