The gas-phase hydrogenation of buta-1,3-diene has been studied with alumina-supported iron (200–260°), cobalt (75–150°), nickel (45–110°), and copper (60–130°), and a series of cobalt and nickel powders (110°) as catalysts. Deuterium has been used as a tracer in reactions catalysed by alumina-supported cobalt, nickel, and copper. Rates of reactions and product compositions are reported as functions of reactant pressures, conversion, and temperature. Buta-1,3-diene was hydrogenated to butene very selectively. Two types of butene composition were obtained: type A in which the yield of but-1-ene was high and the trans: cis ratio in the but-2-ene was low (ca. 1–2), and type B in which the yield of but-1-ene was relatively low and the trans: cis ratio was high (4–11). Iron, copper, and cobalt and nickel catalysts reduced below 350°, showed type A behaviour, whereas cobalt and nickel reduced above 400° showed type B behaviour. Behaviour of cobalt and nickel catalysts could be changed from one type to other. Mechanisms for type A and for type B behaviour are proposed. Over each metal, butene is formed by 1 : 2- and 1 : 4-addition of hydrogen to the alkadiene; no isomerisation of butene takes place before desorption. The type of butene composition obtained depends either upon the degree to which the catalyst is sintered, or upon the completeness of the reduction during preparation.