Gold Cluster Catalysis: A variety of gold salts and complexes have been used to catalyze different organic reactions. Often, the catalytic rates for these reactions are similar. Oliver-Meseguer et al. (p. 1452 ; see the Perspective by Hashmi ) observed an induction period for the onset of catalysis of organic reactions, such as the ester-assisted hydration of alkynes, for different gold salts and complexes. Mass spectrometry and absorption spectroscopy revealed that small gold clusters (three to ten atoms) formed during these induction periods and are likely to represent the active catalysts. The catalytic reaction rates could be extremely high—up to 10 ⁵ turnovers of the catalyst per hour.