A series of nanosized palladium catalysts supported by silica and polymers, prepared by the complex transformation method, were used for catalytic hydrogenation of nitrobenzene. It was found that both the thickness of the polymer layer and the size of the palladium particles would affect the catalysis. The results indicated that the reaction rate would be optimal when a polymer supporter formed a single layer on the silica surface. The catalytic activity of the catalyst would not simply increase with the decrease of the size of palladium particles. The effects of other conditions on the catalytic properties, such as the order during preparation of the catalyst, solvent, and the pH values, were also discussed. (C) 2003 Wiley Periodicals, Inc.