In the presence of a catalyst modifier(an onium salt, an amine, or a polyethylene glycol (PEG)), in a biphasic aqueous-organic mixture, the activity and the selectivity of hydrogenation reactions conducted using catalysts such as Pt/C, Pd/C or Raney-Ni, can change significantly. Hydrodehalogenation of haloaromatics, reduction of functional groups on the aromatic ring and of the aromatic ring itself, benzyl group hydrogenolysis, can be conducted under mild conditions, with high rates and interesting regio-, chemo- or stereoselectivities. The modifier coats the catalyst, thereby forming an interfacial film, wherein the reactions take place. This film partitions the catalyst at the aqueous-organic interface and influences the outcome of the reactions by mediating the substrate-catalyst interactions. (C) 2003 Elsevier Science B.V. All rights reserved.