The selective isomerization of 2,5-dihydrofuran (2,5-DHF) to produce: 2,3-dihydrofuran (2,3-DHF) over 1% Pd/SiO2 requires the addition of low levels of CO to the reaction feedstream. In the absence of CO, the selectivity is approximately 70%, with equimolar amounts of furan and tetrahydrofuran forming the balance of the products. At 50 ppm CO in the feedstream, selectivity increases to >94% with virtually no decline in activity. Higher levels of CO lower overall activity with only marginal! improvements in selectivity. These results, based on:statistical modeling, CO chemisorption measurements, and DRIFTS experiments, are best explained by the preferential adsorption of CO on basal planes, such as (111) facets, on the surface of:the Pd crystallites to change the average ensemble sizes of contiguous Pd atoms. These ensembles are too small to catalyze, the formation of furan and tetrahydrofuran, but not the isomerization to 2,3-DHE The role of CO does not appear to be linked to preferential adsorption of CO on Pd defect sites, such as those at the corners and edges of Pd crystallites. DRIFTS results indicate low concentrations of CO are preferentially-adsorbed on the low index planes, such as (111) facets in different bridged modes. Linearly-bonded CO on edge and corner Pd sites is detected only at levels of CO higher than used during isomerization conditions.