Studying the butyne hydrogenation on Pd and Pt catalysts by the use of transient methods and in situ IR spectroscopy, it was found that the formation of a surface hydrocarbon overlayer regulates the semi-hydrogenation selectivity, It might be composed of eta(2)-butyne species. These species are equilibrated with the reactive intermediates which are thought to be vinylic adsorbed species. However, if the reactive intermediates are initially quickly hydrogenated, the coverage of the surface in equilibrium with the gas fugacity is so low that the hydrocarbon overlayer does not form and the catalyst is not selective, We propose a surface picture for butyne hydrogenation in which the reaction takes place on the low coordination sites (top, edge, corner atoms) while the high coordination sites are covered by spectator species (eta(2)-butyne). It is suggested to transpose this picture to an ethylene hydrogenation reaction in which the spectator species might be di-sigma-bonded ethene.