The hydrogenolysis of the alpha-C-O bond of mandelic acid derivatives catalysed by 5% Pd/C, in the presence of hydrochloric or sulphuric acids as cocatalysts, was carried out in water or ethanol as solvent, under 35-150 kPa of hydrogen pressure, at 343 K. Typically, the substrate/catalyst/cocatalyst ratio was 200 : 1 : 10. The hydrogenation of the ethyl ester of mandelic acid in ethanol as solvent is much faster, ca. 20 times, than that of the acid in water. The influence of the concentration of the reagents, products and cocatalysts on the initial reaction rate was investigated. Upon increasing the concentration of the ester the rate increases to a plateau. The pressure of hydrogen has little influence. The products inhibit the reaction, The rate steeply increases and reaches a maximum upon increasing hydrochloric acid concentration. From equilibrium constant data, the concentration of protonated ester as a function of the hydrochloric cocatalyst concentration has been estimated. The trend of the concentration of the protonated species parallels the trend of the reaction rate, thus suggesting that the protonated species plays a key role in relation to the catalyst activity. It is suggested that from this species, adsorbed on the catalyst surface, a molecule of water is displaced by a hydride formed upon activation of molecular hydrogen by palladium. Though less effective than hydrochloric acid, sulphuric acid acts also as a cocatalyst. However, in the latter case, the initial hydrogenation rate increases to reach a plateau. In addition, when HCl is introduced in the reaction after the preactivation step of the catalyst, the hydrogenolysis rate is equal to the rate observed when sulphuric acid is used as cocatalyst. It is suggested that in the first case the possible formation of the superficial PdOxCly may be related to the higher activity of the chlorided catalyst.