Hydrogenation catalysts prepared from the silica-supported binuclear palladium complexes [Pd2X4(PR3)(2)] (X = Cl, Br or I, and R = Me, Et and, for X = Br only, Pr-i and Bu) and mononuclear complexes [PdX2(PR3)(2)] (X = Cl, Br or I, and R = Me, Et) have been examined throughout their life cycle (induction period, selectively active catalyst and deactivation) by XPS, solid state P-31 NMR spectroscopy and transmission electron microscopy, and their performance and activities compared to a simple Pd/SiO2 catalyst. The binuclear chloro complexes partially react with the silica surface producing some Pd(0) species. The others retain their integrity until contacted by the substrate and hydrogen. The selective solution hydrogenation of cinnamaldehyde to hydrocinnamaldehyde appeals to take place at oxidised palladium sites, most likely dipalladium(I) monophosphine species. The formation of phenylpropanol by some of the catalysts, on the other hand, occurs at Pd(0) sites. The induction period before the catalysts become active results from an autocatalytic process during which the Pd(II) precursors are converted to the active, Pd(I) or Pd(0) species. The catalysts are finally deactivated by a combination of continued conversion of the active species to a new, phosphorus-poisoned Pd(0) material and the restructuring of a hydrocarbonaceous overlayer.