The gas-phase hydrogenation of ethyne over Pd/Al2O3 is studied using the Temporal Analysis of Products (TAP) reactor, leading to valuable insights into the reaction mechanism. The exceptional time resolution of this transient technique enables to capture the hydrogenation of the carbon-carbon triple bond via the sequential addition of hydrogen. Hydrogen/deuterium exchange experiments demonstrate the dissociative and reversible adsorption of H-2 on palladium. Conversion and selectivity change at early stages of the reaction as a consequence of the formation of a stable carbide layer. The alkene/alkane ratio was markedly higher in TAP compared to reported flow data, due to the mbar pressure operation of the technique. In this regime, the formation of subsurface hydrogen is unfavorable, enabling the assessment of the sole effect of reactants coverage on the product distribution, that is, in the absence of unselective hydride phases. Mechanistic aspects related to the use of carbon monoxide as a selectivity enhancer are also discussed. (C) 2012 Elsevier B.V. All rights reserved.