Buta-1,3-diene hydrogenation to but-1-ene, trans-but-2-ene and cis-but-2-ene has been catalysed by evaporated films of the majority of the elements of Groups 3-11 of the Periodic Table. The effects on product composition of H-absorption into the metals of Groups 3-5 and of S-adsorption onto the metals of Groups 6-10 are also described. For the pure metals of each transition series, the but-1-ene yield increases with increasing Pauling electronegativity indicating that an electronic effect governs the overall extent of 1:2-addition. The effect of H-absorption into V, Zr, Nb, and Hf films and the general effect of S-adsorption onto films of the later transition elements is to increase the trans:cis ratio in the but-2-ene which indicates the stabilisation of pi-allylic half-hydrogenated states at positively polarised metal atom sites, Mdelta+. The hydrided and sulphided systems differ in that 1:2-addition by simple hydrogenation of one double bond in the reactant is retained in the former but poisoned in the latter.