The enantioselective hydrogenation of four alpha,beta-unsaturated carboxylic acids has been studied over cinchona alkaloid-modified supported Pd catalyst in a high-pressure continuous-flow system using a fixed-bed reactor. The hydrogenation of the aliphatic substrates resulted in products of slightly lower enantioselectivities as obtained in batch reactors under similar reaction conditions. The optical purity of the product formed in the hydrogenation of alpha-phenylcinnamic acid was over 70%, exceeding the values obtained in slurry reactor in the same solvent. It was shown that the enantioselectivity increasing effect of benzylamine is kept in the fixed-bed reactor. Based on the results obtained by changing the modifier concentration in the feed we suggest that the interaction of the substrates with cinchonidine may occur in the liquid phase and the complex formed is adsorbed on the I'd surface. Thus, relatively high concentration of such complex and consequently modifier amount is necessary for obtaining good enantioselectivities in the hydrogenation of alpha,beta-unsaturated carboxylic acids. (C) 2007 Elsevier B.V. All rights reserved.