DFT calculations were performed to explain the preference for the conversion of trans-2-butene to its less stable cis isomer on Pt(111) surfaces reported previously. The results indicate that the main factors controlling the direction of this reaction are the lesser degrees of molecular rearrangement and surface reconstruction required in the adsorption of the cis isomer on the hydrogen-covered Pt surface. This is a case where hydrogen coverage and atomic restructuring not only dominate the energetics of adsorption but also define reaction selectivity.