Structural changes were induced in Pd and Pd-Ag catalysts by oxidation and reduction. The influence of oxidative restructuring on the selectivity for acetylene hydrogenation to ethylene was studied under conditions of industrial significance (acetylene conversion > 99%). We found that oxidative restructuring of the Pd-only catalyst had no influence on hydrogenation selectivity; neither was any correlation found between selectivity and extent of beta -Pd hydride formation. When Ag was added as a promoter, however, oxidation-reduction treatments had a major impact on selectivity. High-resolution transmission electron microscopy showed rough surfaces were created by oxidation of the catalyst which were coincident with poor selectivity. Annealing the catalyst at progressively higher temperatures in H-2 improved the selectivity to ethylene. Reduction in H-2 at 500 degreesC also suppressed the formation of oligomeric hydrocarbon by-products during reaction. In situ infrared study of CO adsorption showed that the addition of Ag to the Pd/silica results in an ensemble or geometric effect, increasing the proportion of linear CO relative to the bridged form. We infer that oxidation causes the Pd and Ag to segregate within the particle. High-temperature annealing in H-2 causes a migration of the promoter Ag so as to yield improved selectivity on these catalysts. The silica support surface also seems to provide a suitable environment to allow restructuring of the Pd-Ag crystallites to Occur.