The hydrogenation of buta-1,3-diene in the temperature range 45-110-degrees-C at 1 bar (101.5 kPa) total pressure has been studied using the pseudo-binary alloy systems MRh3-xPdx (M = Ce, Zr; 0 less-than-or-equal-to x less-than-or-equal-to 3) as catalysts. Characterisation studies show that surface and bulk Rh/Pd ratios are very similar but the surfaces were enriched particularly in cerium, with strong indications of oxide formation. Similar conclusions can be drawn with respect to zirconium but to a smaller extent. Differences in product distributions, particularly in butene distributions are discussed in terms of two mechanisms which can interpret differences between the Rh-rich system and the Pd-rich system in respect of cis/trans-but-2-ene ratios.