The hydrogenation of pentenes, pentenenitriles and 1-pentyne has been studied over Al2O3-supported Ni and Pd catalysts. In particular the role of hydrocarbonaceous residues in such reactions has been probed. This has been achieved through the use of a tapered element oscillating microbalance to record mass changes in situ during the catalytic reactions with simultaneous analysis by gas chromatography. Sequential adsorption and reaction studies demonstrate that the influence exerted by hydrocarbonaceous deposits is critically dependent upon the compound from which they are derived, with deposits from different isomers of the same compound exhibiting different effects. In particular, in the case of 2-pentenes, a hydrocarbonaceous overlayer resulting from the hydrogenation of cis-2-pentene significantly activates Ni/Al2O3 towards hydrogenation of trans-2-pentene and vice versa. In the case of pentenenitrile hydrogenation however, trans-3-pentenenitrile deposits an overlayer which renders inactive the sites responsible for the selective hydrogenation of cis-2-pentenenitrile to pentanenitrile. Results from 1-pentyne hydrogenation studies over Pd/Al2O3 Support previous conclusions regarding the development of a carbon-rich active phase. This work demonstrates the importance of the consideration of heterogeneous catalysts as dynamic materials which evolve during reaction and adds to the increasing awareness of the potential positive roles played by carbonaceous deposits in such reactions. (C) 2008 Elsevier B.V. All rights reserved.