The effect of the metal particle size and the modifier concentration on the enantioselectivity (ee) and the conversion level in the hydrogenation of three different substrates: 1-phenyl-1,2-propanedione, ethyl pyruvate and acetophenone were studied over Ir/SiO2 catalysts. The metal loading was varied from 1 to 5wt%. The catalysts were characterized by H-2 chemisorption, TEM, XRD, TPR and XPS. The reactions were tested in a stainless steel batch reactor at 298 K and 40 bar, and all experiments were performed using cyclohexane as solvent and cinchonidine (CD) as chiral modifier. The conversion and ee increase with the metal particle size and the number of surface ensembles and the catalytic results revealed that 5 wt% Ir/SiO2 catalyst reaches higher conversion and enantiomeric excess (ee) in the three studied reactions. Additionally, the relationship between the activity and ee with CD concentration, exhibit a volcano type curve and the three studied reactions, indicative of the importance of competitive adsorption between the modifier and the substrate on the catalyst surface. (C) 2008 Elsevier B.V. All rights reserved.