In the presence of a catalyst modifier (an onium salt, an amine, or a polyethylene glycol), in a biphasic aqueous-organic mixture, the reactivity of Pt/C, Pd/C, and Raney-Ni changes significantly. Hydrodehalogenation reactions of halo aromatics, reductions of functional groups on the aromatic ring, benzyl hydrogenolysis, can be conducted under mild conditions, and with interesting rates and regio-, chemo-, and stereoselectivities. The modifier coats the catalyst, and forms 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 substrate-catalyst interactions, and by providing a hydrogen reservoir in the vicinity of the active sites. (C) 2002 Elsevier Science B.V. All rights reserved.