Preparation of nanosized palladium (Pd) catalysts supported on various carbon supports using a simple liquid-phase reduction of aqueous Pd complexes with potassium borohydride (KBH4) was investigated. We found that addition of appropriate amounts of sodium hydroxide (NaOH) into aqueous solutions of sodium tetrachloropalladate (Na2PdCl4) followed by reduction with KBH4 produced highly dispersed Pd nanoparticles less than 5 nm in diameter on any carbon supports. Ultraviolet-visible light (UV-vis) absorption spectra of aqueous Na2PdCl4 solutions in the presence of various amounts of NaOH clarified the occurrence of partial or complete ligand exchange of the Pd complex from chloride ions (C1(-)) to hydroxide ions (OH-). Hence, the production of small Pd nanoparticles on carbon supports is likely to be induced by structural change in the precursor complex prior to the reduction with KBH4. Moreover, evaluation of catalytic activities for liquid-phase oxidation of benzyl alcohol into benzaldehyde using oxygen and hydrogenation of the C=C bond in cinnamaldehyde with hydrogen revealed that catalytic functions were significantly dependent on the size and the dispersion of Pd particles; i.e., both reactions proceeded efficiently on small Pd catalysts highly dispersed on carbon supports but did not occur efficiently on Pd particles of relatively large sizes or with poor dispersion on those supports. (c) 2006 Elsevier B.V. All rights reserved.