The hydrogenation of but-2-enal over supported Au catalysts is discussed, together with a detailed characterisation study using X-ray diffraction, infrared spectroscopy and transmission electron microscopy. Au/ZnO catalysts are found to be selective for the formation of the unsaturated alcohol, but-2-en-1-ol rather than the saturated aldehyde, butanal. In general, the addition of thiophene is found to enhance the yield of the unsaturated alcohol. Detailed transmission electron microscopy and infrared spectroscopy studies show that thiophene modification of Au/ZnO catalysts does not affect the Au-particle size or morphology; rather, thiophene undergoes irreversible dissociative adsorption giving a surface in which the Au sites are electronically promoted by sulfur. It is observed that thiophene modification does not give any marked effect on catalyst performance for the catalysts that contain large Au-particles (10 nm) and, hence, it is considered that the sulfur promotion observed is associated with smaller Au nanoparticles. The highest but-2-en-1-ol selectivities (∽80%) are observed for 5 wt.% Au/ZnO catalysts reduced at 400°C prior to reaction. It is proposed that the origin of high selectivity is associated with large Au particles (10–20 nm in diameter) that are present in this catalyst.