Propyne hydrogenation over alumina-supported palladium and platinum catalysts at 293 K has been investigated under pulse-flow conditions. Catalytic activity and selectivity is shown to be critically dependent on hydrogen concentration. An equimolar C3H4:H-2 mixture was 100% selective to propene for the Pt/Al2O3 Catalyst, whereas the Pd/Al2O3 catalyst produced both propene and propane in comparable amounts. Correlations in activity, carbon mass balance and variations in the product distribution for excess hydrogen mixtures indicate distinct regions of the catalyst conditioning process, as the catalyst approaches steady state operation. The active site for propene formation is attributed to a hydrocarbonaceous overlayer, formed during the early stages of the reaction sequence. Comparisons with comparable studies on silica-supported palladium and platinum catalysts suggest that the metal-support interaction plays a role in defining the hydrogen transfer characteristics of the overlayer. Furthermore, it is tentatively suggested that, for supported Pd catalysts, the degree of carbon retention under reaction conditions is structure sensitive, with larger metal particles exhibiting a greater degree of carbon laydown. (C) 2003 Elsevier B.V. All rights reserved.