A Pd/SnO2 catalyst was doped with low concentrations of Bi in order to induce selective poisoning of sites on the Pd surface in order to test a hypothesis that specific surface sites are responsible for ammonium formation during the catalytic hydrogenation of nitrates from drinking water. Initial addition of low levels of Bi appeared to selectively titrate high-energy surface Pd atoms located in the high index planes of the Pd crystallites. Addition of Bi had little effect on catalyst activity but modified selectivity profiles of nitrite and ammonium formation. (c) 2006 Elsevier B.V. All rights reserved.