The cis/trans isomerization of 2-butene on Pt(111) in the presence of hydrogen has been studied by means of calculations based on density functional theory (DFT). Two hydrogen precoverages have been considered, 0.11 and 1.00 ML. The di-sigma bonding geometries are the most stable and the trans isomer is preferred at the low hydrogen precoverages, whereas the pi bonding geometry of the cis isomer is the preferred structure at H saturation coverage instead. The set of hydrogenation/dehydrogenation reactions that leads to the conversion from one isomer to the other, which involves an adsorbed alkyl intermediate, has been studied for both coverages, and the transition states for those steps have been identified. The differences between the activation energies-of the reactions involving the two isomers are small, but a different behavior can nevertheless be observed depending on the hydrogen coverage considered. At low hydrogen precoverages, conversion from the cis to the trans isomer was estimated to be preferred, whereas on the hydrogen saturated surface the reverse appears to be true. Surprisingly, an Eley-Rideal mechanism was identified as the lowest energy pathway in the latter case. The results from this work are consistent with previously published experimental data from temperature programmed desorption and infrared absorption spectroscopy studies. (C) 2013 Elsevier Inc. All rights reserved.