The selectivity of 1-butene hydrogenation/isomerization on Pd catalysts is known to be particle size dependent. Here we show that combining well-defined model catalysts, atmospheric pressure reaction kinetics, DFT calculations and microkinetic modeling enables to rationalize the particle size effect based on the abundance and the specific properties of the contributing surface facets. The selectivity of Pd catalysts in 1-butene hydrogenation and isomerization depends on nanoparticle size. Herein, the authors show by combining experiment and modeling that the type and specific properties of the contributing surface facets are decisive for the selectivity.