Selective semi-hydrogenation of 1,3-butadiene has been investigated in excess of hydrogen over Co, Cu and Pd-Ni catalysts. The results on 5 wt.-% Co/Al2O3 and Pd-Ni/Nb2O5 catalysts provide evidence that hydrocarbonaceous materials formed on these surfaces play an important role in governing the competition between 1,3-butadiene and n-butenes. Similar results have been observed on 5 wt.-% Cu/SiO2 modified by preadsorption of n-butylamine. On the poisoned surface hydrogenation and isomerization of n-butenes have been remarkably inhibited, but the sample remains active in semi-hydrogenation of 1,3-butadiene. It is proposed that the unusual high selectivity of alkene formation is ensured by firmly held adspecies (FHA) formed from diene or n-butylamine. FHA cover and modify metal sites and do not allow adsorption of n-butenes as they apparently cannot compete with FHA. Hydrogenation of diene has been interpreted by a compression-displacement model, in which the high strength of diene complexation plays a definite role. Butadiene ’pushes away’ FHA for reason of its high complexation strength and finds (create) reaction sites for its semi-hydrogenation. Preadsorption of CH3OH on 5 wt.-% Cu/SiO2 affects the region selectivity of n-butenes : on the modified surface the selectivity of cis-2-butene increases at the expense of 1-butene. These results have been interpreted by electronic modification of Cu sites by intermediates formed from decomposition of methoxy species.