Thiolate-protected Au-n clusters are exploited as homogeneous and supported catalysts for two chemical processes: i) the selective oxidation of styrene by O-2 to benzaldehyde and ii) the chemoselective hydrogenation of alpha,beta-unsaturated ketone to alpha,beta-unsaturated alcohol. The results demonstrate that a quantum-size effect in Au-n clusters plays a critical role in their catalysis. This work highlights the importance of pursuing quantum-sized gold clusters for catalysis. [GRAPHICS] .