The enantiomeric excess (ee) in asymmetric hydrogenation may vary with the concentration of hydrogen dissolved in the liquid phase. This concentration is proportional to hydrogen pressure when the reactor operates in chemical regime, but depend on the stirring rate if mass transfer is limiting. For example, decrease of ee from 61 to 51% ee has been observed upon varying the stirring rate from 100 to 2000 rpm. A good characterisation of the reactor used is recommended to quantify the influence of hydrogen on ee. (c) 2006 Elsevier B.V. All rights reserved.